I'd add that this is not nearly the same as riding the hammer on a double action pistol when reholstering, since in that case the hammer should be in the de-cocked position. In the case of a 1911 if somehow the safety mechanisms fail and some event occurs that would cause the hammer to fall and the only thing keeping it from doing so is your thumb, as soon as you release the hammer the gun is going to fire (unless you become aware of it and take measures to prevent it). Of course such a failure is unlikely and would mean something really bad happened like an internal parts failure -- but probably not impossible.
My apologies if this discussion is muddying up the thread.
Personally I put gun on Safe and ride my thumb under the hammer while reholstering. A bit different than when I aiwb my P2000 I then actually do place my thumb on top of the hammer.
With a beavertail I can't ride the hammer without seriously compromising my grip. I lock my thumb under the safety; it may be hubris but I feel like I have enough reps to know if the safety isn't on when my thumb is in this position. Most of my holsters have a feature which won't allow the pistol to be seated with the safety off, so I essentially "hand off" to this feature.
That makes sense, my holsters have the same feature. But based on other comments I guess I was assuming the purpose of riding the hammer on a 1911 would be in case the safeties fail mechanically somehow and that causes the hammer to fall? Perhaps I assumed wrongly and the real reason is in case the thumb safety is not in the on position and somehow the trigger is pulled -- in that case riding the hammer would keep it from falling (at least temporarily) and might also cause the grip safety to engage since you pretty much lose your firing grip.
So far I am not in the habit of riding the 1911 hammer during reholster, but I have not converted to AIWB yet.
Out of curiosity, what grip are you trying to maintain when holstering? This could certainly be a function of hand size, but for 1911's I usually take the web of my hand off the grip to ensure the grip safety is active, thumb the hammer while holding the frame curled under my fingers and holster as such. Like you, I generally only use thumb safety-engaging holsters.