Should you file your taxes jointly — or separately? Answers straight from CPAs

First comes love, then comes … taxes?

When you're in the honeymoon phase of a relationship, you're more likely to be dreaming about how many kids you'll have — not how your sweetheart could affect your future tax bill.

But once relationships start to get serious and the idea of marriage has been tabled, understanding just how taxes work for married couples should be a key part of joint financial conversations.

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While we aren't advocating that you spend too many date nights talking about deductions and credits (could there be a worse way to kill the mood?), many newlywed couples can feel caught off guard once tax season rolls around — even long-marrieds who've had a sudden change in their financial situation.

To explore just how complex paying taxes as a couple can be, we asked CPAs to take a look at three hypothetical scenarios and offer up advice for how each pair could navigate their tax filing conundrum.

Because it's not just the love bug that bites — so can paying Uncle Sam.

The newlyweds with starter salaries

Chloe, 26, and Alex, 25, got married three months ago. She earns $45,000 as a social worker, while he makes $35,000 as a grant writer for a nonprofit.

As newlyweds, they're trying to figure out the best way to do their taxes. If they file joint taxes, Chloe stays in the same 25 percent tax bracket but Alex would move up from the 15 percent to the 25 percent bracket.

Chloe also started saving for retirement for the first time this year, and has successfully maxed out her $5,500 maximum IRA contribution. Alex hasn't yet started saving for retirement, but Chloe thinks she can spare $3,000 of her own money to allow him to open an IRA.

Because they've been married for less than a quarter of the year, they're wondering if they have the option to file as singles for the last time — or at least as married filing separately — so that Alex can stay in a lower tax bracket.

In addition, Chloe is unsure of whether she can help contribute to Alex's IRA because she's already reached her contribution limit.

What the CPAs say: The I.R.S. sees no distinction between those who were married for one day versus a full year. "It's your status as of December 31, 2014, that counts," says Gail Rosen, a CPA based in Martinsville, N.J. So Chloe and Alex are considered married for the 2014 tax year.

When it comes to their retirement savings, the good news is that Chloe can give Alex the $3,000 to open his own IRA because that money counts toward his contribution limit, not hers. This not only enables them to tackle retirement as a twosome, but the IRA contributions help lower their joint taxable income — which, in turn, reduces their tax burden overall.

But if they file jointly, will the resulting bump in Alex's tax bracket have a negative effect on this year's returns? The short answer: no — at least when it comes to the couple's federal taxes.

According to Rosen, if Alex and Chloe went with a married filing separately status, Chloe would pay $3,953 in federal taxes, while Alex would pay $2,829 — for a combined $6,782.

Filing jointly, on the other hand, works out to slightly less, for a total of $6,776, which makes filing jointly the seemingly better choice — by a nose.

Married filing jointly is almost always the better option for couples because "the tax brackets for married filing separately go up pretty fast," says Brian Newman, a CPA with CohnReznick in Hartford, Conn. "The higher rates kick in at lower [income] amounts."

However, other factors could swing newlyweds similar to Chloe and Alex toward the married filing jointly track — and one of the top reasons is back taxes.

"One reason I often see [for married filing separately] is that, just before getting married, one spouse ran up tax debt," says Robert Wheeler, a CPA in Santa Monica, Calif. "That means the non-owing spouse would be responsible for that debt too."

The high-earning couple with high medical bills

Sarah and Sam, both 38, have been married for 12 years, and have a daughter, Lily. They both have advanced degrees and are doing well in their careers, although their salaries diverge widely.

Sarah, a physical therapist at a private clinic, makes $68,000, while Sam earns $180,000 as an attorney with a large commercial firm. Until now, they've always filed their taxes jointly, but medical expenses incurred from a cancer scare has them rethinking their strategy this year.

In early 2014 Sarah learned she had stage 1 cancer, which required scores of diagnostic tests, surgery and radiation therapy. The good news: Since it was caught early, she's expected to make a full recovery. The bad? Even with health insurance picking up 80 percent of the costs, her out-of-pocket expenses still came to $12,000.

Sam, on the other hand, has had no health problems and his medical expenses — co-payments on routine doctor and dental visits — barely add up to $200 annually.

According to I.R.S. rules, you can itemize medical deductions that exceed 10 percent (7.5 percent if you're 65 or older) of your adjusted gross income (AGI). With their combined income of $248,000, Sarah's $12,000 of medical expenses would fall well below that 10 percent threshold, even with other deductions the couple estimates they could take to lower their AGI.

But that $12,000 is 17 percent of Sarah's salary, and could provide a decent tax break if she were to file on her own. They want to know whether they should file separately, so that she can itemize the medical deductions on her taxes and, hopefully, lower their overall tax burden.

What the CPAs say: Sarah and Sam's situation is a little less black and white than Chloe and Alex's, and whether they file separately will depend not only on Sarah's medical costs but also on the other deductions they'll be itemizing.

In some cases, married filing separately limits which deductions or credits a couple can take.

For example, when married couples file separately, the rules allow only one spouse to claim the tax exemption for a child, says Rosen. If Sarah alone were to take the exemption for Lily, based on her tax bracket, and deduct the aforementioned medical costs to lower her adjusted gross income, she would owe $8,988 in federal taxes.

Based just on Sam's hefty salary, Rosen says Sam would owe $45,071 in federal taxes if he filed separately, bringing the couple's combined tax burden as separate filers to $54,059.

And what if they filed jointly? They would owe $49,000 — a savings of more than $5,000. In the end, Sarah's medical expenses weren't enough to convince them to file separately.

But, as Newman points out, this outcome is based on the fact that they're itemizing only one major expense — the medical bills — and most couples in their demographic would likely have many more deductions.

"If they have other itemized deductions — home mortgage interest, charitable contributions, real estate taxes — then [filing jointly] may not work out better," he says.

In Sarah and Sam's situation, crunching all the numbers with a CPA would be the best way to truly understand what would save them the most in taxes — especially considering that "when a married couple files separately, they either both have to use the standard deduction or both itemize," Wheeler says.

This essentially means that the more deductions a couple has to work with — particularly if they are large ones, like major medical costs — the more options they'll have for creating a better tax scenario.

The domestic partners who are new parents

Michael, 40, and Keith, 42, have lived together for seven years. Michael makes $85,000 as a public relations executive, and Keith takes home $100,000 annually as an online sales director.

Although they consider themselves married, they reside in a state that doesn't allow them to be legally wed, so they are registered as domestic partners.

In 2014 they adopted a son, Tommy, and started a side business from their home refurbishing old furniture.

Until now, their finances — and taxes — have been largely separate. But they're wondering how their joint costs and income will impact how they file their taxes moving forward.

What the CPAs say: As domestic partners, Michael and Keith can't file a federal tax return as a married couple under I.R.S. rules — but this can work to their benefit because it gives them flexibility in how they divvy up potential tax breaks between them.

For starters, only one of them can claim Tommy as a dependent, and typically, "the person who gives more than 50 percent of the child support usually takes the deduction," Rosen says. However, if they are contributing equally to Tommy's costs, "it might be more advantageous for the lower-paid person to take the deduction," she adds.

The one thing they can share, however, is the adoption credit, which enables them to lower their taxes by up to $13,190 per child to cover qualified adoption expenses.

They can either deduct the costs they were responsible for — for example, perhaps Michael covered the agency's fees, while Keith paid for the attorney and court costs — or they can split the credit however they choose, as long as the combined amount they are claiming doesn't exceed what they actually paid or the $13,190 limit.

As far as the income from their side gig goes, how they divvy up business-related deductions, such as costs for a home office or special refurbishing tools, depends on the percentage of the business each owns, says Newman. "If it's 50 percent each," he says, "they each deduct half the expenses and must each declare half the income on their taxes."

One thing that could throw much of these rules out the window, however, is if Michael and Keith live in a community property state. This is a state that grants couples equal ownership over any income earned or property purchased during a marriage — and Nevada, Washington, and California fold domestic partners into these rules.

For domestic partners, this could actually be a disadvantage because they'd be required to split their combined income in half for tax purposes. "So you may be responsible for taxes on your partner's money," Wheeler explains.

This is similar to the so-called marriage penalty many couples deal with — they find themselves owing more taxes than they did as single people because their combined incomes push them into a higher tax bracket.

For this reason, getting married eventually may not save Michael and Keith money on their taxes. "People who never marry often pay less in taxes because of the marriage penalty," Rosen says. "Congress has tried over the years to make it better for two working people to get married — but it's still not better."

And Rosen speaks from experience. "Thirty years ago I put my wedding off until January to save $1,200 [in taxes]," she says. "It paid for my honeymoon."

This story was originally published on LearnVest. LearnVest is a program for your money. Read their stories and use their tools at LearnVest.com.