We tip the same at buffets as regular sit downs. Buffet servers at Disney may do not "bring your food" but they do everything else, including keeping an eye on your drinks, keeping your dirty plates cleared, orienting you to the buffet, and at character meals they will help you time things right so you don't miss the characters. They receive the same low wage (way below minimum wage) as the other tipped positions so I think for good service they deserve the 20%.