I always tipped my groomer because I am paying for a service. I tip my own hair dresser so why shouldn't i tip my dogs?
Now, i just recently became a certified dog groomer and i will say i'm so happy when clients tip. I think it shows they are happy with the groom and the services provided. Plus right now i'm not making too much money since i'm new, so tips are greatly appreciated as non taxed cash in my pocket.