I have been an accompanist for many open mic'ers (esp. on the harmonica and/or harmony vocals) and I NEVER go up unless I am asked. I have done lots and lots of session work, but I always look at my job sort of like Steve Cropper does (Booker T. and the MG's, for the uninitiated): He is a session player par excellence and a great songwriter; he NEVER gets in the spotlight on another person's gig, though. he's there to back THEM up.

Just my thoughts.

Peace on you,

'Bear