My wife and I had a discussion yesterday, and we've decided that we're pretty much done with hatchling rescues. During hatched we're pretty much going to do our best to maintain proper incubation temperatures and humidity, but during the hatching process, we're going to let nature take it's course. For example, if a chick can only pip and not zip, we're not going to assist.
From our experience, most of the hatchlings that need assistance have some kind of issue and need post-hatch treatment and/or therapy. Quite often they die anyway or have to get a long in life with a serious challenge. For example, we had three pre-hatch rescues for this last hatch. Two of the chicks have since died, and the remaining one has seriously curled feet and a distended navel.
We have some rescues in the past that have made it and are perfectly healthy, but they tend to be the exceptions. Anyway, it wasn't an easy decision, but after going through this ritual so many times, it seemed the best decision for us.