The thing about pipping and hatching and helping that people need to realise is this: Just because a chick has pipped, doesn't necessarily mean it's actally ready to hatch out yet. Chicks don't pip when they're ready to hatch, they pip when they're running out of oxygen inside the air cell and they're starting to suffocate. So they pip to reach fresh oxygen, but they may not be fully developed and ready to start hatching for a few hours, or maybe even a couple of days...
If you're too quick to 'help' out a healthy chick, you could do it a lot of damage. If your temps and humidity have been spot on, you shouldn't have to help out ANY healthy chick. If you know you've had some temp and/or humidity issues, you may have healthy chicks struggling to hatch through no fault of their own, and in that situation helping out might be necessary. But if you've not had any such issues and you've got a chick struggling to hatch, chances are there's something wrong with it and nature's not intending it to hatch.
I'm just saying all this to put across the idea that helping chicks out shouldn't be the norm like some people seem to think it is. When I started incubating, I helped out quite a few chicks. I know now that most of them would probably have managed to hatch by themselves if I'd (a) got my temps and humidity stable and (b) just left them alone. In the last two years, I haven't helped out a single chick, and every chick that has pipped in my bator has gone on to hatch unassisted.