The size of the cut does make a difference in my opinion, There is a big difference between making a small slit and cutting away a large portion of the top of the egg. I don't cut as often these days however I still do cut any non-pipped eggs. I don't cut until at least one of the eggs has pipped however often before noticing that one egg they all have pipped so I don't have to bother. All eggs that have not pipped I cut. Yes I have had full term babies dead in the egg and some of those I checked did not have an egg tooth, it either didn't develop or it was knocked off too early. I can only assume that it drown in the egg trying to free itself, I don't know ANYBODY who would have a necropsy done on a stillborn hatchling so you're never going to have PROOF of it but I think it's a reasonable assumption.
Neal; I use scissors to cut my eggs. No matter how careful I try to be with a razor I'd often still go too deep and cut some blood vessels (although this is not as critical with a baby the size of a ball python as it is with smaller species). You don't jab the tip of the scissors into the egg, you hold the scissors sideways and cut a small nick in the egg, then you slide the tip of the scissors inside the egg keeping the tip tight against the inside of the egg shell and make a small snip, this way you can usually avoid cutting any blood vessels. I generally only cut a small V shaped slit about an inch long or so, just enough for a babies head to get out. Usually they don't even bother using it and make they're own cuts but sometimes they do.
Has this saved the lives of some babies that couldn't get free on their own? There really is no way to PROVE it, but I think it has as I've noticed a few hatchlings come out by way of the cut I had made without making any cuts themselves.