In my experience, trying to help a chick out is usually a mistake.
The only time I didn't regret doing it is when I had a chick that pipped at the wrong end of the egg, and I waited a LOOOOOOONG time to make sure he was ready before helping him.
But generally speaking, I don't think there are many good reasons to "help" a chick hatch.
After YEARS of refusing to "help" chicks hatch (due to bad experiences "helping" eggs hatch when I was a kid) I finally gave in (against my better judgement!) and "helped" two eggs this spring.
The first had started hatching, had gotten almost 3/4 around the egg, then stopped. Since it was a late hatcher I thought maybe the poor thing simply didn't have enough energy to finish. It was an egg that I REALLY wanted to hatch, so I decided I would "help" it. BAD choice! The chick knew EXACTLY what it was doing -- it had stopped hatching because the yolk sack wasn't absorbed! It died several hours after I "helped" it. I felt absolutely terrible!
Another egg had been pipped for over 2 days, but showed no sign of hatching, and the peeps coming from within were getting steadily weaker, so I "helped" it. Again, big mistake -- it was blind in both eyes, had a deformed skull, and its legs were crippled beyond repair.
As a kid I helped eggs hatch that genuinely needed it, as the chicks really were too weak. Initially I prided myself, but later these birds were poor layers, got sick easily, and died early. They were weak birds and (perhaps this sounds cruel to some) probably should have died when they couldn't get out of the shell.
I'm sure plenty of people have "helped" their chicks hatch to great success, and ended up with birds that were perfectly healthy. But somehow I suspect in many of these cases, the "helping" was probably less necessary than percieved, and the chick was simply getting ready to hatch anyhow.
I'm not trying to put anyone down or say that anyone is wrong, but I do feel that I need to say something. Anyhow, these are my experiences and I hope people read it and take these things into account before trying to "help" their chicks hatch.