I cut once the first one pips, and take a peek to look for problems. A twisted umbilicus can be fixed, has nothing to do with a snake's genetics or fitness, and it would be a shame to lose an animal I could have saved from that.

I've found no higher rate of adverse incidents in snakes from cut eggs than in those that pip on their own. (Actually, the ones that tend to leap out of the egg too quickly are usually ones that pip by themselves).