My general opinion reading these posts and some that I read after googling yesterday is that it is a normally tipped position, but groomers are not upset if they don't get a tip.
I have 5 dogs and have paid $75. per dog, all the way down to $40 per dog for grooming. I did buy all the equipment and did it myself for a couple years after one of my dogs was injured at Petco and started growling at anyone who brushed his back. I didn't enjoy the grooming and neither did my dogs and good clippers are very expensive. I'd buy the cheap ones for around $100. and they would last a few months.
I started out using Petco and there was no reference to tips and I asked some other people who got their dogs groomed there and they all thought I was nuts for even considering tipping. Of course, that was 8 years ago. Maybe things were different back then.
I had the mobile groomer for a while ($75/dog) and times that by 5 dogs and I didn't tip, except at Christmas where I'd give a significant tip. She had a baby and didn't go back to grooming. Her replacement was a complete train wreck and I ended up going back to shop grooming.
The place I mostly use now has a grooming room without any cages. The dogs just walk around and hang out together. It's a very social environment. The grooming isn't the best and since they have several groomers, I can either get a really good groom or a really bad groom, but my dogs don't mind going there and they are in and out quickly because they don't cage dogs between things that they do. It's much more like when I was doing it at home. They ask for tips, but they also pool their tips. I've been giving $5 per dog. I quit taking my blind special needs dog there because I think it is too stressful for him.
I'm taking my special needs senior to a new place next week. The shop owner is going to do him and it's the first appt. of the day which costs $10. more, but it is a straight through groom and doesn't involve cages. The estimate is $62. I wasn't sure if I should tip him or not. I think after all my online research (found a few forums where groomers answered), I should actually tip him $5 or $10.