It absolutely depends on the type of jam, and the type of music, and the individual song. And the subltey or lack of it with which one plays. If it works, it's cool. If it don't, it ain't.
I took a tambourine to a bluegrass jam years ago when I didn't know nuttin. I'm completely surprised I lived over it. I know now what a close call it was because I've gotten to know a bunch of musicians since then and I've rarely known bluegrassers to be so restrained.
But for gawd's sakes if someone is trying to do a sensitive, quiet solo, stash the shakey stuff. Of course, as a singer who actually does try to sound relatively decent during jams, I could say the same thing of about 12 of the 15 guitars in the circle. At least don't keep flailing away at them at full volume when you've got a singer about to bust a vocal chord to sing over you.
there's a guy in our musical circles who plays musical saw. It's fun every now and then, but he doesn't seem to read the social cues that say, "Enuff."
--ww