As Peter says, it depends on the type of music and the culture of the session.

Some Irish tune sessions in particular are very rigid, and you're expected to play the tune their way - if it's a tune you don't know, rather than joining in by ear you're expected to go away and learn it, and not join in until you've got it right.

English music sessions, on the other hand, tend to be more relaxed, and playing about with melody and rhythm is encouraged.

Both approaches have their strengths and weaknesses. The former produces some very tight and well-played music, perhaps at the expense of some spontaneity. The latter can produce some amazing music from the right group of musicians, but can also go horribly wrong!

It should go without saying that you must adopt the approach of the particular session you're playing in.