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If you clench your fists and gnash your teeth at the very thought of paying anything extra while flying, the idea of tipping at 35,000 feet might seem absurd.
Yet, the issue regularly comes up as travelers wonder whether it's appropriate or might make a flight more pleasant.
In a recent poll on Airfarewatchdog.com, more than a quarter of respondents—27 percent—said they had tipped a flight attendant at some point during their travels, whether as a thank you for doing a good job or for going out of the way to make them more comfortable.
Etiquette experts caution it's not expected or necessary, and the Association of Flight Attendants says its members don't accept tips.
(Read more: High times: Booze is big on Vegas flights)
But crews say they do sometimes see fliers offer cash and gifts.
Heather Poole, a veteran flight attendant for a major U.S. carrier, said it doesn't happen often, but she has been tipped on flights.
She sees the practice most frequently on Las Vegas routes. Most passengers who try to tip are in economy class, she said.
"It's company policy not to accept tips. That said, it's always appreciated when a passenger makes such a nice gesture," said Poole, the author of "Cruising Attitude: Tales of Crashpads, Crew Drama, and Crazy Passengers at 35,000 Feet."
"Normally, I'll turn it down at least three times, but if someone continues to insist that I accept a tip and then shoves it in my hand or pocket, I might take it. At this point, I feel like it's almost rude not to."
A regular passenger on the New York-Los Angeles route once gave the crew gold hoop earrings during Christmas, Poole recalled. Most tips consist of "a couple of singles," but there's been a time when a passenger presented $50, she added. Many people might be shocked at how little some of her colleagues make, Poole noted.
The median annual salary for a flight attendant was $37,740 in 2010, according to the latest figures from the Bureau of Labor Statistics, though an entry-level position might start at $16,000.
Still, it's not a job that requires people to tip, said Lizzie Post, an etiquette expert and spokeswoman for the Emily Post Institute. A flight attendant's first priority is to maintain the safety and security of the passengers in the cabin, noted Corey Caldwell, a spokeswoman for the Association of Flight Attendants.
The profession is not listed on PayScale's most recent Tipping Study, which ranks occupations that typically receive gratuities from customers, such as waiters and valets. Fast-food workers and tow truck drivers make the list, but not flight attendants.
"Unlike servers in a restaurant, they are paid a livable wage and it's important to recognize that difference in why we tip and why we don't," Post said. "We don't tip everyone who is in a service position that helps us out."
She couldn't think of a situation where it would be appropriate to tip a flight attendant and suggested travelers simply don't do it.
(Read more: Drink up! Wines fly free on Alaska Airlines)
If you want to recognize outstanding service from a crew member, Post advised contacting the customer service section of the airline's website and writing a complimentary note that includes your flight number and that airline employee's name.
"A nice letter means more than a lot to a flight attendant," Poole agreed. "In (the) airline world people are quick to complain, so a few kind words really do go far. Sometimes those letters actually affect our careers."
Saying "please" and "thank you" also makes an immediate difference because flight attendants don't hear those words very often these days, Poole added.
"You wouldn't believe how much a passenger with nice manners stands out on a plane," she said.
(Read more: This country claims to have the best tippers)
—By A. Pawlowski, NBC News.