I don't cut my eggs unless I suspect a problem, but then I am more about the natural order of things. I once had a somewhat dried out egg on top of the pile, so I cut it after another egg in the clutch pipped. Thankfully, the baby inside was perfectly fine, just a tad on the small side (though she did catch her siblings quickly).

My own rule of thumb has always been to allow the eggs to pip on their own, but if all other babies in a single clutch have exited the egg but there is still one yet to pip, I will cut it to check on the baby inside.