In the two open mikes I periodically take part in, the sign-in sheet is put out before things start. As referred to by GUEST,Susan above, one can sign up for a slot in the proceedings, not limited by when you're signing in.

I try to avoid going first or second, because the audience is apt to be rather thin as people straggle in, and I don't want to be late in the order of things because people start to leave. I'd rather sing for a larger crowd than I'd get in the early or late slots.

However, sometimes the MC has asked something like, "Uncle Dave, Charlie broke a string and has to change it; could you take his 'first' slot?" or "Uncle Dave, Charlie signed up for first, but he's disappeared somewhere.? I know your guitar is out and tuned, so would you start things off so we can keep on schedule?" Of course I'll do that for the good of the cause, and never a mumblin' word about my not-first druthers.

Dave Oesterreich