I have a strict rule about it this and I always insist my semi-pro and amateur musician friends charge something for performing. That way I'm hoping venues will appreciate the work that goes in to putting on a performance. I'm also against doing auditions for gigs too. Surely a Youtube clip should suffice. I know some musicians send out highly produced demos that are not representative of their actual live sound, let's say, but there are a lot of us who work to perfect our live performances. Those people going out half rehearsed, half prepared and half baked are giving us hard working musicians a bad name.
But I'm going off on a tangent, a bit of a personal moan. I wanted to make a point about whether it is ever appropriate to perform for free. I know I have done it in the past but where does it really get you? Possible leads, a follow up paid gig? Actually, in my fourteen years as professional musician I can honestly say it's gotten me nowhere.
Unless you are helping out a friend - don't play for free. We work hard (a lot of us) and have trained for years and it's about time we got respect for our art and stop being told to get a "proper" job. Even those of us who are not famous are hard workers. Let's celebrate our creativity and get paid for it - we're worth it!