One of the most generous tippers (percentage-wise) I ever knew was a former waitress.
That's generally the case. I waited tables years ago, and therefore I've always tipped generously, I know what they go through.
It should be a requirement in life to work in the service industry at least once in one's life.
I knew one guy who immediately after being seated would put 20 one dollar bills on the table, and pointed out to the waitress that was her tip, but if she screwed up, one by one he would remove dollars from the table for each screw up. When being served a drink, he would tell her, "I don't want to see the bottom of this glass."
Of course, that is always they case. They were part of the racket themselves!