You want at least about .003" or it may tend to bind when the cylinder heats up or you get a bit of carbon fouling. Other than that, the tighter the better. I personally think a new gun shouldn't be more than .006 or .007...but I'm told that S&W now considers a BC gap up to .012 "in spec" on their new revolvers. I consider that crazy because my old Model 19 with thousands of rounds through it is at about .009 or .010 and side-spitting quite a bit of powder with full .357 commercial loads...enough so that at an indoor range the burning powder bounces off the lane dividers and hits the side of my face! (Needless to say, that gun doesn't get full-power commercial loads any more.)
However, I have seen tests that indicate that a widening BC doesn't cause as much velocity loss as some folks think...though I suspect that's more true with short barrels than long ones...haven't seen empirical data to back that assumption up.
John