Paid Time Off: Why You Should Use It or Lose It

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Got your PTO yet? If not, chances are you will soon.

More than half of all companies offer employees PTO, or paid time off, instead of a set number of vacation days, sick days and so on. If you're never quite sure how to count time off for religious holidays or children's soccer tournaments, this promises to simplify your life.

Also, if you're hankering for a long weekend, and July 4 happens to fall on a Thursday—well, taking off July 5 suddenly looks a lot easier.

Paid time off may have another effect, though. If your sick days, personal days and vacation days all draw down your bank of free time, you may be less likely to take a day off when you are a little under the weather.

That could boost your productivity—but it could also put you and your co-workers at risk of getting truly sick.

"Employees are making choices before they decide to call in sick," said Evren Esen, manager of the Society for Human Resource Management's survey research center, which has looked at the prevalence of paid time off banks.

(Read More: Which Country Gets the Most Vacation Days?)

Saving paid time off can work well for you if your employer lets you hold on to all your unused days. But a new survey by SHRM found that companies are less and less inclined to let you do that. Just 9 percent of employers allow employees to cash out unused vacation time when they leave their jobs, down from 13 percent in 2012 and 18 percent in 2010.

"It may be decreasing because of economic reasons. It does cost extra," said Esen.

(Read More: No Paid Vacation? You Must Be an American)

No kidding. At BART, the San Francisco area's transit system, where workers are now on strike over issues including wages, a group of senior managers have accumulated a combined total of 69 years—yes, years—of unused vacation and holiday time worth nearly $8 million.

The agency's former general manager, who resigned under pressure in 2011, was BART's highest-paid employee last year despite not working at all because she was having her unused time paid out, according to the San Jose Mercury News.

(Read More: San Francisco Area Rail Strike Causes Traffic Chaos)

Plenty of people wind up with unused time off for good reason, of course. You may be unusually hardy, or you could be working hard on a big project, and finish the year with extra days.

But with fewer employers allowing carryovers of unused time, you have less and less reason to hoard it. After all, if you can't use the time later, why wait?

Teachers in Toronto appear to have figured that out. After the provincial government stopped allowing school employees to save unused sick time, teacher absences spiked 22 percent over the previous year.

Esen stopped well short of recommending that you stay home on July 5—but she did acknowledge that saving days may not be your best plan.

"I would say especially for vacation time, that employees should utilize that time because the organization is giving them that time so they can rest and relax and come back to work refreshed," she said. "It's not required by organizations in the U.S. to provide vacation or sick leave or any paid time off. It is a benefit that employees should take advantage of."

Sounds like it's time to pack your beach bag.

—By CNBC's Kelley Holland.