Tipping may be the most confusing travel behavior of all. Haven't we all wondered about when to tip, whom to tip and how much? Let's debunk five myths to help guide you on your next trip.

1. There's no need to leave money for a hotel housekeeper.

The median hourly pay for U.S. hotel housekeepers is $9.21, a spokeswoman for the UNITE HERE union says. UNITE HERE housekeepers in Las Vegas get $16.19 an hour and $20.94 in San Francisco, but those are exceptions. Housekeepers may work for the minimum wage, and most travelers don't leave tips, they say. But consider the job. On a typical day, a staffer may be responsible for 15 or more rooms. Have you scoured 15 bathrooms, changed 15 sets of sheets, and vacuumed 15 rooms in seven or eight hours? Try lifting those heavy pillow-top mattresses that now adorn hotel beds. I once worked alongside a housekeeper and couldn't do that task solo. Gladis and I sweated through 17 rooms and received not one extra dollar.

Etiquette experts suggest $2-$5 a day, depending on the luxury level of the hotel. I leave cash daily under the pillow so there's a better chance it ends up in the hands of the cleaner who did the work. Since some properties have policies against taking money left in a room, write "thank you" on a piece of paper or put the cash in an envelope.

2. A restaurant server will receive the same amount whether you tip in cash or on a credit card.

Not necessarily. Some eateries make waiters and bartenders pay the credit-card company service fee on the gratuity (often 2%-3%). Employers have even demanded that servers pay a percentage of those entire tabs, which isn't legal.* And servers may not get credit-card tips the day they're given, which means a wait and the chance for employers to fudge on what is owed.

As for what to leave: "20% is what people say they are tipping," says Peter Post, great-grandson of etiquette doyenne Emily Post and managing director of The Emily Post Institute. "They do it because it is easy to figure out." If that's too rich for your blood, 15%-18% is OK, especially at informal eateries, Post adds. As for the debate over whether to tip on the entire bill or just the before-tax amount, Post says you don't have to add in the tax. But he tips about 20% on the total "wherever I am. It's just simplicity." Most diners use the bottom line as a tip basis, say my server friends. However, check that a service charge hasn't already been included: Places catering to tourists from countries where tipping isn't the custom increasingly tack on an extra percentage. And many foreign countries already include a service charge on the bill, so read it closely.

3. The hotel concierge should be paid for every reservation or arrangement she or he makes.

While concierges do rely on tips to boost their incomes, they don't necessarily expect cash for simple directions or a quick phone call for an easy dinner reservation. It's considered best to hand over an envelope at the end of your stay as thanks for a seat at the theater, sporting event or a chic eatery. "For 95% of things, $5 or $10 is fine," Post says. He'll go higher for tickets to pro playoff games, for instance. If a concierge recommends a bad restaurant or theater seats are less than billed, you'll be sorry you tipped before the event. I once gave a Paris concierge five euros for a reservation at an empty, tacky bistro that I'll bet was giving him a kickback.

But should a concierge pull off a complicated feat such as a wedding proposal involving skywriting or a scavenger hunt, you might want to break out a Benjamin.

4. Those room-service fees go straight to the staffer.

Don't count on it. Bills are hard to decode, and hotel policies vary, say experts at the STR hotel research firm. A "delivery charge" or "delivery fee" typically goes to the hotel (room service is not a moneymaker for most lodgings, says Jan Freitag, an STR senior vice president). A "service charge" is usually a tip, but it might be pooled among the room-service team, and in some cases the hotel, and can be less than 20%. In addition, there's a line on many room-service checks for an extra gratuity that's hated by road warriors on tight expense accounts. This generally goes directly to the server, and "if the service is awesome," many travelers leave a few dollars more (often in cash to be certain the server gets it), but they don't have to, says Freitag. If you want to reward good service, ask the server what percentage he or she is getting. The whole situation is awkward when you're in a bathrobe trying to do the math, Freitag says. He thinks travelers would welcome more clarity and simplicity.

5. I have to accept the automatic daily gratuity added to my bill by the cruise line.

Cruisers used to routinely hand over envelopes containing tokens of appreciation for stewards, waiters and others at the end of a sailing. Some lines still follow this practice or include gratuities in the fare (such as Seabourn and Azamara Club Cruises). But today, most of the biggies tack on extra dollars a day to ensure that low-paid employees get them. But you don't have to accept the amounts. Go to reception or the purser's office and ask that totals be adjusted downward - or up. Think before scuttling tips, though. Remember that, rightly or wrongly, many doing the more menial jobs on the ship rely on tips as the mainstay of their pay.

*This year, Philadelphia-based Chickie's & Pete's chain agreed to pay current and past employees about $8.5 million over a situation like this. The judgment also covers inadequate wages paid.

Freelance writer Kitty Bean Yancey is a former USA TODAY travel reporter.