As a banjo player, I play mainly in the standard gDGBD tuning, and can play easily in G, C, D, and related minors. Dmin, F, and A come easy enough as well. Slap a capo on it, and you're within a few frets of any key. The fifth string takes 2 seconds to change, either spike it or get one of those sliding thingies to capo it. Whoever needs to stay in one key all night is probably not a very good banjo player...