It totally depends on the venue and occasion, AND the sit-in. I have had many great ones, a few bad ones. I'm sure you've noticed that the good ones don't ask; you have to ask them.
My rule is they have to know the words and the key. NO KARAOKE.
I've been known to break my own rule for a C-note.
Eddie, the same guy has yelled in my ear before! Once it was so bad I had to just stop in the middle of the song and apologize to the audience that I couldn't hear myself. His friends quickly came and got him.
Back in the late 70's/early 80's I did a piano bar (except I used an organ, later a keyboard). It was very lucrative, but a true pita. Never again!
DonM