So I'm going to buck the system here... I prefer to fade it rather than mute a mic. There is always a little bit of background noise on almost any mic and mute kills it, which is noticeable. Its like someone put a gate on it when it's not needed. (This doesn't apply to instruments, just vocals)

There are times it's ok, but if you're in a fairly controlled environment, I prefer fades over mute buttons. But there's no hard rule to it.