Extenders can congratulate themselves or their accountants on actually filing the form, on time, that gives them six months to file the final forms. They still have to pay estimated taxes, however—something Greene-Lewis said some late filers are shocked to discover. If you owe, whatever you pay has to be within 90 percent of your liability or you’ll pay a penalty.

Extenders are a predictable bunch. “Most of the people who file later typically procrastinate every year, even if they’re getting a refund,” said Greene-Lewis. And, yes, many wait until right around the six-month deadline, she said. By September of 2015, only about one quarter of all taxpayers who filed for an extension had filed their taxes, said Martha O'Gorman, chief marketing officer for Liberty Tax Service.

Some taxpayers have little choice but to file an extension, often because they need a form called a K-1. These forms, filed by private equity, venture capital, and hedge funds structured as partnerships, can take until late summer to show up, as the partnerships themselves may have filed for an extension. (They do have to file by Sept. 15.) Waiting for forms needed under the Affordable Care Act may also force people to file late in the game .

The Players

Some people aren’t procrastinators but owe the IRS and don’t want to pay any sooner than they have to. So they schedule their return to be filed close to the deadline.

Many people buy a little more time—and get reward points—when they pay their taxes using a credit or debit card. “Our users tend to want to pay online using a credit or debit card because they may earn points or rewards of cash back,” said Sheri Chin, vice president of marketing with ACI Worldwide, the largest payment processor for the IRS. “Depending on when they have to pay their credit card bill, they may get extra time to pay.”A Credit Sesame survey of 500 of the site's users found that those who planned to put their tax bill on plastic are doing so mainly for the points—54 percent chose that as their reason, compared with 27 percent who said they didn't have a solid emergency fund or savings. Eighty-six percent of those charging their taxes say they will pay it off in the next billing cycle.

The Oldsters

The older you get, the more likely you are to file on the last day possible, according to a recent Jackson Hewitt survey. Only 3 percent of millennials said they would wait until the last minute, compared with 10 percent of baby boomers, and 13 percent of those age 70 and higher.

Millennials, with their simpler finances, win the early-bird prize, with 28 percent saying that "they filed the minute I received my W-2." About the same percentage said they try to file early to get their refund. None of them planned to file an extension, compared with 2 percent for the other demographic slices.

The Rest

Then there are the people who aren’t procrastinating but just don’t know they can get money back by filing. Their income may be under the IRS filing threshold of income of $10,300 for a single person and $20,600 for those married and filing jointly, and figure there’s no reason to file .

That’s not necessarily true. “A lot of college students have money taken out of their paychecks that could be eligible for some tax credits, maybe refundable ones,” Greene-Lewis said. They may be eligible for an Earned Income Tax Credit (EITC) of up to $503 for 2015 with no kids. They may also be eligible for the American Opportunity Tax Credit, worth up to $2,500; 40 percent ($1,000) of that is refundable, she said.

Her own nephews were working while in college and not filing taxes—until she told them that if they got federal taxes taken out of their paycheck, they should probably get money back. Now they file.