Originally Posted by Hair Trigger

I've not seen that advice, but I think it is horse pucky. Just as a test, I pulled out all three of my 1911's to double check where my thumbs are when I have a one- or two-handed grip, and they're well away from the left-side thumb safety (I'm right handed), even with the oversized ones on my Ruger SR1911 and my Springfield EMP4. Even less an issue with my Springfield A1's original-sized safety. Mainly for the reason you mention, not getting sliced by the slide.

It's like anything else in shooting; practice hand position, practice keeping your finger out of the trigger guard, practice thumbing the safety on or off, practice keeping your thumb away from the safety .....practice, practice, practice until handling the gun is automatic (semi-pun intended).