From what I've read, it seems that tangs are not poisonous, while rabbitfish/ badgerfish are.
Tangs have a scalpel-link spine at the base of their caudal fin that they erect when frightened or trying to defend themselves. These can inflict painful wounds into other fish, but most tangs will swim away rather than attack you.
Rabbitfish, however, have poisonous spines at the front of their dorsal fins that can bring some numbing pain wherever they puncture the skin. Again, as long as you keep your hands out of the aquarium as much as possible, you should not have to worry about being stung.
Hope that helped.