This disorder is associated with several vascular, neurological, and skeletal problems including stroke, heart attack, clotting, mental retardation, eye abnormalities, and osteoporosis, amongst many other complications.
Tattoo tipping etiquette, in general, should hover around 15% for small tattoos and 20% for larger tattoos, but as with all tips, you as a customer ultimately determine the amount based on the quality of service and the work performed. Consider your involvement as well.
It all depends, most tattoo shops charge by the hour, $100 and hour is average but some go right up to $150 or $200. But if it is a large peice they may charge you in sessions which you would have to ask a store about.