I always tip my groomer. I take my babies to the groomer once a month for EVERYTHING (which cost $40 per dog) -- bath, wash and blow, trim, nail cut, anal glands expressed (ewww), ears cleaned and ear hair plucked, etc. They get the works!!
Then in between visits (basically 2 weeks after their complete grooming), I bring them in for minor touch ups -- nail cut, ears cleaned, hair around feet and toes trimmed and hair by butt and pee pee shaved.
She usually never charges me for this but I always give her a $20 because I really do appreciate it.
I give her a Christmas bonus too. She's just awesome and treats my dogs so great. I'm also allowed to sit there and watch the whole process.