Light a/c concencus is yes turn it to sby incase you select 7700, or 7600 or 7500. It is only these that make the warnings flash up. E.g. have flown lgw departures with a 77xx squawk before.
Airliners (757 / 767 at least), leave it on, as turning it off, even momentarily turns the tcas off completely - this could be fatal. Just be careful what you are dialling up.