I have no idea whether it is proper etiquette, but if I was getting a tattoo I certainly would suggest, BEFORE THE ARTIST STARTED, a tip could be had for great work. After all, you don’t want them to accidentally, say, misspell your girlfriend’s name to that of a former girlfriend (a couple of years ago in NYC that happened, first the guy w/the tattoo assaulted the tattoo artist, then the guy’s girlfriend assaulted the guy, then she assaulted the artist, then the guy sued the artist).
I also suggest tipping your dentist, urologist, gynecologist, obstetrician and proctologist.