Your healing response is slowed while on Accutane and scarring is increased, so it's something you really ought to wait for, but as you can see, lots of people pull it off. Some of this depends on the location of the piercing you want, and some on the skill of the piercer.
I would personally not get a cartilage piercing while on Accutane since those are the hardest to heal due to low blood flow, or any of the hard to heal piercings like navel, or anything surface-to-surface. (Obviously some people manage to heal these while on Accutane, but i would wait.) Someplace like the tongue should be fine. Just be extra careful with your cleaning regimen to avoid infection. And a word of caution about tongue piercings - your dentist will hate you! I wore one for about ten years and wore a nice circular groove where my front teeth meet, both top and bottom, because i was unconsciously biting it. Once it heals - take it out at night. Be aware that they can also give you headaches if the top bead rests on a sensitive spot on the roof of the mouth.
Make sure your piercer autoclaves the instruments between uses, and your needle should be fresh and never used on anyone before - it should be removed from the packaging in your sight.
