You don't have to tip but it's considered rude to go to a restaurant and not tip unless you received terrible service, tipping in a service industry is proper etiquette. A good number to work with for tipping is 10-15% but if that's to steep on a larger tattoo even $20 a session is better than nothing. Your artist splits all the money they make with the shop, they lose anwhere from 40-60%. They often have to supply all their own inks and supplies as well as machines, power supplies and accessories which can be anywhere from $500-$1000 per machine setup.
The tips they receive they keep 100%
Please always tip apprentices, typically apprentices work for 6months to sometimes over 2 years with no pay, your tips are the only thing that they get and are often the only way they can succeed in becoming a tattoo artist or piercer.