I've never thought of tattooists as a profession for tipping.
Example: I'm getting work done now. I originally laid out a $100 deposit for a half sleeve tat back in October, and made three 3hour appointments to have it done.
Then in March we decided to buy a house, and I felt bad dropping over $1200 on a tat when the money could be used on stuff for the house. So I compromised and decided to spend half my tattoo budget on a tat, half to the house stuff. I went back with a new design and said I had a budget of $600.
Now, the tat I'm having done is pretty complex. It's based on an abstract oil painting of a phoenix. My artist said she'd take a couple days to decide if she could do it in my budget. After a couple days, she said she could and she set the size. I even told her I thought it was a bit big, but she was confident the size was fine.
Then the work started. The woman is a fastidious and dedicated artist, so she really put her 110% into translating the painting into a tattoo. The complexity of color laying even surprised her, and after the first 3 hour session, she only had 1/3 of the tat done (she used over 20 different colors in the first session alone). The second session was just as slow. I've had six hours of work and it's only half complete. But she said "I'm not worried about the price. Pay me your budget and that's fine."
Even then, I slid her an extra $50. Call it a tip. Call it gratitude for doing a $1200+ tat for $600. Call it whatever. Would I tip if she did a $600 tattoo for $600? Probably not. The way I see it, if you really love their work, you're going to go back them again and again for more, and will tell all your friends where to go for good work. The tip is increased and continued business.