From Tricks of the Trade:
Most gigs involve playing multiple sets, e.g., you go on at 9, play til 9:45, and go back on at 10. If the audience sees you looking at your watch, though, they will assume that you are bored. So wear your watch so that the dial is on the inside of your wrist. This way you can discretely glance at the time while playing.
This site has some valuable tips and tricks for musicians. Check it out.