Perform to the best of your ability. Sometimes I get the feeling that people come along to a sing-along with a "that'll do," attitude. In other words, they've sung or played it through a couple of times, and "that'll do." Sorry, but it doesn't. If you're going to perform in public, always prepare your material and do the best you can. Having to listen to someone fumble and bumble their way through a piece of music with constant hesitations, wrong notes, and re-starts, is not pleasant for the listener and does nothing for the confidence or reputation of the performer. Do your rehearsing at home - not in front of an audience.