I think it's a legitimate question (I'm in my 50's, and haven't gotten a tat in many moons!). Your good tattoo artists of today are skilled professionals who demand and receive top dollar for their work - an hourly rate beyond most plumbers and electricians, for example, and comparable to many lawyers. (A LOT more than I make per hour, I assure you!) I don't tip those folks when I retain their services.
The guy doing my tattoo tomorrow has been at it for 20 years, and is the business owner. He's very successful, financially speaking. It strikes me as slightly silly - perhaps even a tad insulting - to offer him a tip as if he were some kid waiting tables, or a girl cutting hair. It's rather like tipping a partner in a law firm who's done some legal work for you.

All that being said, I will tip him, and hope he doesn't snicker at me...!