If you can't SAFELY lower the hammer of a semi-auto pistol on a live round, you have to question your ability to have one.
Why? Most semi-autos that are meant to be carried with the hammer down on a live round have some way of decocking them safely. It seems to me an unecessary risk to lower the hammer on a live round manually.
As far as 1911's, you'd NEVER want the hammer down on a live round in a series 70 gun. You could do it with a series 80, but why would you?