See the space in front of the case neck, in this Sheridan cartridge gauge?
That is the clearance I have between the case neck and throat, and I want enough so that drive in, and unpredictable events, don't push the case neck into the throat and pinch the case. If the case neck is pinched, because the neck is sticking in the throat, that will create over pressure conditions.
I have trimmed a lot of cases well below OAL, I have shot a lot of 30-06 sized down to 270 Win. A neck sized down 30-06 is so short I don't need to trim after neck resizing. And I cannot tell much of a difference on paper, out to 300 yards, in my hunting rifles.
Now maybe some bench rest shooter, or F Class guy will see something on paper, but their equipment will hold inside anything I can do with a hand held weapon. So shooters saying it will affect neck tension and that will affect group size, need to put targets out there, with more than three shot or five shot groups, to prove this. Trim length is primarily a safety consideration, and I do it, or check, everything time the rifle brass is reloaded. I also trim new brass before loading, just to make sure.