1. Single action 1911 with the hammer cocked- Safety is needed, because the firearm can be discharged by applying to the trigger a very small force over a very small distance.
2. Double action revolver with the hammer not cocked- No safety is needed, because the firearm can not be fired by applying to the trigger a very small force over a very small distance.
3. Striker fired semi auto- Same determining criteria as above.
Other determining criteria- a) If I have an overriding large concern that a perp can grab my firearm, or that I could have a negligent discharge, add a safety to the revolver and add an additional cascading safety to the single action 1911, and add a safety to the striker fired semi auto.
b) If I have an overriding large concern that the safety could impede a need to immediately discharge my firearm, then no safety on the firearm.
If my other criteria a) and b) are about equal and cancel each other out, I see 1., 2., and 3. above.