As a tattooist; if I feel like I did a good job and my client tips me, it's an awesome feeling and I gladly accept the tip. If I did a tattoo that I felt I could have done better on I refuse the tip.. Why should I be paid extra when I didn't knock it out of the park?
As a collector; I always tip.. wether its after each session or after its all said and done. Typically, as I'm getting tattooed I talk to my artist and find out what s/he is passionate about other than tattooing.. Wether it's firearms, music, cars etc. I don't tip until my multiple session piece is done then I drop a rad gift on them.
In my opinion, I would rather hand my tattoo artist a new guitar, a new rifle, or a new set of tires.. rather than handing then $50 tip every time.