People do still ask me if you are supposed to tip a tattoo artist. (I know, right?) YES. You are.
People also ask me how much is appropriate. Well, the guy just gave you something that you will have for the rest of your life, one of the few things that can’t be lost or stolen. So not tipping him is kinda like flicking your waiter in the left testicle for getting your order right. Not cool. Don’t be that asshole.
As far as how much is appropriate, it should be in some way proportional to how much you spent and how much you love it when its finished. Be good to your artist, he’ll be good to you. Unless he’s a dick, in which case find another artist.