Tip if you like the groom. If they do a horrible job, then by all means, don't do it. If your pup is difficult, then tip.. It's very hard to make a puppy look good. They wiggle and have bad hair to work with.
Here in Canada people tip for all types of services like dog groomers, hair dressers, gardeners, repairmen, mechanics, and other service type jobs. But unlike a waiter or waitress it's not expected. I'm a groomer and about 1 out of 4 people tip. They're so nice
"Most" of my clientele tip...like probably 5 out of 7 tip, and these are pretty decent tips too. I enjoy these tips, but never expect them. Clients tip if they like the groom, and they also are more likely to tip if the groomer is hospitable and friendly to them as well as their pet.
That said, I am a very, very meticulous type, so even a dog who get's stripped down with a 7 blade I make sure he looks smooth, and spend as much time on the face, to ensure that extra 'cuteness' factor, being the body is 'gone'. A dog never leaves dirty, and I hate those dogs that you just can't get all the grease or scaley skin off of, because I feel like I haven't done my job well enough. I am also really good with nervous, nasty dogs and cats, so that boosts some of my standings with clients.