IMO, anything that may be carried cocked (SA) needs to be carried cocked & LOCKED.
Otherwise, if it can be carried DA hammer down, a passive automatic/ internal firing pin block or other firing pin safety should be adequate.
So... 1911s, HPs, CZs, etc (and some HKs) really NEED a manual safety (downward sweeping) IMO.
Sigs need none (and have none); I wouldn't consider them necessary on an M9/ Beretta 92 either.