I was curious to see what the 'official line' was on this subject- so I went to
www.fodors.com (great site by the way) and copied this out of the WDW guidebook
Tipping
Whether they carry bags, open doors, deliver food, or clean rooms, hospitality employees work to receive a portion of your travel budget. In deciding how much to give, base your tip on what the service is and how well it's performed.
In transit, tip an airport valet $1-$3 per bag, a taxi driver 15%-20% of the fare.
For hotel staff, recommended amounts are $1-$3 per bag for a bellhop, $2 per night per guest for housekeeping, $5-$10 for special concierge service, $1-$3 for a doorman who hails a cab or parks a car, 15% of the greens fee for a caddy, 15%-20% of the bill for a massage, and 15% of a room service bill, which is often already included, so be sure to check.
In a restaurant, give 15%-20% of your bill before tax to the server, 5%-10% to the maître d', 15% to a bartender, and 15% of the wine bill for a wine steward who makes a special effort in selecting and serving wine.
I was glad to see that they did not distinguish between buffets and other restaurants, as I have always adhered to the above guidelins- and sometimes give more. If you think about it, it makes sense that it wouldn't be a lower percentage- as the total food bill is generally much lower at a buffet
Blessings,
mic_key_mouse