I successfully helped about 8 due to crashing humidity around the Halloween Hatch. All of them are doing well so far and none seemed weak or sickly.
I think if you are careful about who you help and why, then are careful about HOW you do it, you can do some good. If it's the fault of the conditions at hatch time, I definitely try to help, because it's not that the chick was weak or shouldn't hatch then, it's because I couldn't keep the sponges damp enough. When you are helping, go slowly and observe what you have. Break egg shell at the air cell and not inner membrane. If you can see yolk, stop and wait. If you see blood vessels, stop and wait. Set the egg up so that the opening is pointing up (an egg carton is good for this). When you have the chick exposed (membrane pulled back and not too tight around him), leave him be to climb out on his own when he's ready.
I never pip an egg for a chick and I only intervene if something I've done or haven't done or wasn't able to do was at fault.