I always take a number of percussion instruments to the folk club - ours is mainly singing rather than session tunes, but, as MMario says, everything depends on the tune/song being performed. Common sense should prevail. Sometimes a few of us will join in on guitars, while others join in on eggs, tambourine, claves or bodhran. On the other hand, a quiet or unaccompanied song or air should not be "percussed" (if that's the right word). A lot depends on who's leading the song too - if they're not all that confident, don't play along in case it puts them off, even if it's a lively number. And if I go to a club/session I've not been to before, I don't even get the percussion out of my bag until I've sussed out whether it's acceptable or not. If others are joining in with instruments, it's likely that percussion is acceptable - but watch out for facial expressions of disapproval, and be prepared to just clap your hands.

Joan