If you didn’t know that tipping a tattoo artist is customary, YOU DO NOW! Tattoo artists DO NOT get paid an hourly wage, they earn a percentage of the money you pay for your tattoo and that’s it. They depend, heavily, on tips to support themselves.
You wouldn’t NOT tip a waiter unless the service was really bad, would you?
Your tattoo artist is marking you for life, something far more serious than even a fancy meal, so hasn’t he/she earned at least the recognition that a server gets? If you receive exceptional service, show your appreciation by tipping. As a general rule, tipping 10% is a MINIMUM, 15-20% is normal and anything more is considered tipping heavily. If you don’t have the extra money to tip, at least explain that to your artist (they’ll understand and appreciate it), if you can, bring them a tip the next time you have a few extra dollars.