We tipped extra at pretty much every meal - like the previous poster, we have small children and felt like we needed to make up for some of the mess (though we always were under the table picking up as much as we could ourselves).
Sometimes we probably tipped too much - especially at Kona and WCC when our servers were giving my daughter the 3rd degree about her age "and how old are you princess?" DD "I'm 2 (holding up 2 fingers smooshed together)" CM "how old?? 3? (trying to trip up my little girl like we were lying about her age)" DD "NO - 2!!" DD will be 3 on the 23rd, so she pretty much looks 3 (let's face it, she pretty much is 3! but still 2 until the 23rd! and therefore 'free' as far as Disney is concerned). I know I'm getting

here, but this REALLY bothered me (to the point where I'm tempted to write Disney about it....in addition to telling them about all of the wonderful CM's we encountered last week).
Anyway - we pretty much always tipped extra.