From Tip20

Tipping Standards - United States
Note: There are no hard and fast rules about tipping, nor are there laws requiring tipping in the United States. The list below are generally accepted guidelines.

Waiters/Waitresses: 15% - 20% for average - good service
More for exceptional service.
Headwaiter/captain: May get a cut of table server's tip; so tip your server extra to reward captain, or tip captain separately.
Bartenders: $1 - $2 a round -or- 15% to 20% of the tab, with a minimum of 50 cents per soft drink, $1 per alcoholic drink
Cocktail Waitresses: $1 - $2 per drink
Sommelier or wine steward: 15% of cost of the bottle
Buffet Servers: At least $1 per head if you get your own beverages. If you order beverages (or more) from the server, then you should tip 10-20% of the bill based on service quality.
Coatroom attendant: $1 per coat
Room Service: 15% - 20%
Valet Attendant: $2 - $5 average for each trip to the car, more may be appropriate given conditions. (Like weather or location)
Pool Attendant: $1 - $2 for each service (providing towels or lounge chairs)
Showroom Maitre d': $1 - $2 for preassigned seats
For unassigned seating, you may tip according to where you want to sit (usually a tip over $50 will guarantee your seat)
Washroom attendant: 50 cents to $1
Taxi driver: Varies depending on locality. Assume 15% will be enough; an extra $1 to $2 for help with bags.
Food delivery person: The Greater of 10% of the bill or $2.00. Should tip 15%-20% for a difficult delivery.
Grocery loader: Check with store policy if tips are accepted. If so, $1 for bringing bags to car; $1.50 to $3 if you have more than 3 bags.
Barber: 15% to 20%, minimum $1, for a haircut. For other services (shampoo, shave or manicure) tip $1 to $2 to service provider.
Hairdresser: 15% to 20%. (It is acceptable to tip owner, unless he or she says otherwise.)
Shampoo person: $2
Manicurist: 15%
Spa service: 15% to 20%. If service is provided by owner, no tip.
Coffee/food retailers w/ tip jars: Tip is optional.
Handyman: Tip is optional.
Gas attendant: Tip is optional.
Skycap at airport: $1 per bag if you check-in curbside; $2 per bag if skycap takes bags to check-in counter.
Hotel doorman: $1 per bag for help with luggage; $1 per person for hailing a cab
Hotel bellhop: $1 per bag for bringing luggage to your room (but a $2 minimum if you have just one bag)
Hotel housekeeper: $2 to $5 per night
Hotel concierge: $5 for getting you tickets or reservations ($10-plus if they're hard to get). No tip required when you ask for directions.
Cruise: Varies. Ask cruise line about customary gratuities.
Movers: Tip is optional.
Tattoo/Body Piercing: 20%. More for custom or difficult work. Tattoo touch-ups min. $10, Jewelry insertions min. $5.00
The general rule is that you do not need to tip on tax. It has been our experience however, that most people just look at the bottom line (tax included) and just tip on that.