I tip based on how much they need to do. One of mine got matted, so I pre-tipped the gal who does shampoo and comb-out so she would gently take care of the matted hair. She deserved it for the extra work and it was a little extra incentive to be gentle.
Usually they don't have that so I just tip the lady who cuts their hair. I've tipped extra when I wanted something extra, like cutting down from floor-length hair. Takes more time, I'll give them extra. Once, they looked especially great and I was just overwhelmed when I saw them, so I tipped extra then for a better than usual job. So, it is not the same each visit. They seem surprised each time I tip, and to tell the truth, I never tipped before reading it in a post on YT a few years back. Maybe I'm the only one doing it in this small town? IDK? But it made sense when I read it and I have been doing it ever since.