I've heard it reacts unusually with overclocks. Could cause what would otherwise be a stable overclock to be unstable, or it could just plain not work. You're much better off disabling it if you're planning on overclocking.
You don't have to, but I would.
You don't have to, but I would.
What he said.
You don't have to, but having your CPU downclock when idle can/could cause instability issues when OC'ing.
Well then leave it on. We aren't telling you what you have to do, but we are telling you what we would do, and that's what you asked.
I'm not saying to do anything. I am just giving you my advice. I would personally leave cool 'n quite off when OCing, but I am not saying you HAVE to do so. It might cause stability problems because cool 'n quite changs the clockspeeds.