I think I'd tip a groomer if they did a great job. I tip the lady who cuts my boys hair (both of them) because my youngest is fidgety and she deserves it because his hair comes out nice anyway. I don't tip my groomer, but all she has to do is clip nails and my dog is good for it - if she had to bathe him she'd get a tip...normally I get a bath when he does and I don't generally want one at that time.
Our grocery baggers and such get tips (on the base) because they work for tips only and I feel bad for them, they have to go and help you load groceries and such no matter what the weather, etc...if I have a LOT of groceries (in the $300 range worth)I give them about a $5 tip or sometimes more if the weather sucks and I'm dragging them out into the cold to help me load them. Granted, if they hated it that much they don't have to work there, but all the same.
Waitresses/waiters is a different story all together. If the service is OK, I give the 15% that's customary...if it sucks, they don't get a tip, but if they are attentive and not too annoying (you know the one that comes by every 5 minutes and interrupts your conversation) then they get a good tip in the range of 50%. Or, if my kids make a mess that's bad I give them a better tip for putting up with my kids garbage (sometimes we just have those days where there is no stopping some of the crap they do, but we try to have them behave in public)