Assuming you are in a country where tipping is customary in the service industry, you should tip your piercer or tattoo artist. As far as how much, that's difficult to answer, because there is no formula like there is with dining.
The most important thing is that the artist knows how happy you are (assuming you are). It's a tough job, and this helps. Many people maintain that personal gifts are good, especially if you have gotten to know the artist over a number of sessions. Do they have a sweet tooth? Bring them cookies. Of course, a few dollars for a good piercing never hurts, either.
There's no shame in letting people know that it's O.K. to tip. Put a labeled tip jar out.