Interesting question! I sent it to my uncle & will post his answer if possible. He served in an SP field artillery unit in the ETO 1944-45.
As for me, I'm like many other posters here, in that I served in the military in a more modern era (1981-2005) & not in combat. Among other things, I ran security on a nuke weapons site in Germany & emphasized muzzle control a LOT. . .but it was also obvious that it was more than my soldiers had experienced before
We got a lot of "up & downrange" and "finger off the trigger" on the range, but units, soldiers & their leaders varied greatly when not in a range environment.
In my environment, we had a German entry controller wound himself seriously while "clearing" his P1 (aka P38). The German troops also remembered very well that they had an MG-3 (aka MG42) ND'ed into a platoon formation a couple of years before I got to that unit in '83, and 1-2 killed and 2-3 wounded in the incident. A fellow officer, serving on a US-Italian nuke site, lost a US soldier killed by an Italian entry controller's ND (fast-draw practice gone wrong).
In other words, my experience was (a) modern, and (b) applied in an environment where ND was a far bigger threat than hostile action by terrorists, etc.
With all that said, I observed that well-disciplined units, especially combat arms units, had a higher standard of safe weapon handling that went along with their obviously high comfort level with the weapons. I would bet that a similar dynamic shaped the combat troops of WWII, in that they would use their weapons with great familiarity if experienced and well-trained in a good unit. Hastily-trained replacements were notorious in that war (and I think in every war) for getting themselves/others hurt in every possible way, however. What I was trying to do in my unit was to get my people accustomed to safe but competent/confident weapon handling in peacetime, rather than simply waiting for the Grim Reaper to teach us the hard way if we ever went to war.
One poster questioned the use of safety catches while on patrol. . . .I've read a bazillion memoirs (I'm a history type & taught it at USMA for 6 yrs), and lots of evidence indicates that the guy(s) in the very forefront make their own decisions about that, but that anyone not in extreme threat of immediate, close-range contact with the enemy tended to keep his weapon on safe. Too much chance of AD otherwise.
Like others, though, I look forward to anything we may hear from actual combat vets of the WWII era.