Remember that most of the time a "tip" is divided among the wait staff and the kitchen staff. Also there is a certain amount that is "assumed" by the Tax Code a wait staff will be paid and it is figured into the withholding. I generally over tip, more than 20% . I have known too many young people who were working their asses off while in school or otherwise in a low paying job. The hostess who brings your take out is not paid much, and actually gets a lower cut of the tips. Most wait staff would rather wait tables that hostess because they can make more in tips to augment their low pay. When you think about the abuse they have to put up with dealing with the public, I think they are due more than their position would suggest.
When I say take out I mean from a regular restaurant, not a fast food place like Jack's. If it is a meat and three with a steam table, I generally leave $1-2 because they bring me water, generally carry my tray to the table and get me anything I need. If I get great service especially in a place I will go back to I want them to remember me and take care of me so I might go a little more than 20%.

I usually tip 20% at sit down restaurants unless the service sucks, I tip the girl that cuts my hair 2-3 bucks, the two restaurants I usually get take out from get a couple bucks on a order.

Was told a long time ago that TIP was an acronym for "To Insure Promptness". Now how true that is I don't know but it sounds good