We completely help if they've pipped but made no progress after 12 hours. We assess what the problem is (we've had experience with the following issues while hatching):
1) Shrink wrapped. Was our first hatch and the hydrometer was off by 20%! We opened enough of the egg and slammed up the humidity so that it was able to get itself out the rest of the way. The chick is now a healthy strapping rooster.
2) The chick was a stargazer. It had a Thiamine ( Vitamin B1) deficiency due the breeder's birds lack of proper supplementation. She wasn't strong enough to fully unzip so we helped her enough that she could get out the rest of the way. We gave the chick proper support nutritionally and by her 2nd day, she was perfectly healthy and is now a huge lovely Lavender Orp layer.
3) The chick was enormous! Too big for the egg. He wasn't able to get past pipping (over 12 hours), he didn't have enough room in the egg to maneuver into position to unzip. We ended up removing a full half of the shell and rolling him out of the egg. He's now a HUGE Lav Orp rooster named "Hot Fuzz"...he was the fluffiest of all the chicks.
Sometimes it's our error that prevents a chick from hatching. When we help, we do just enough so that we can see their beak and that they are able to breathe. After that, we try and let them do it themselves because if they can, we know they are normal, healthy and strong, they just needed a little hand. It's a value judgement to help or not.
We look at it this way -- if we were in the same situation, we'd want someone to give us every chance possible. And if the chick truly isn't healthy, it will die after it hatches anyway so what's wrong with making sure a healthy chick didn't die in the egg because of some odd ball situation like human error.