Like Renee said, I wait 24 hours after a pip to go in and help. There are some signs that make the case more urgent, such as pipping on the wrong end of the egg. On my last hatch I had a disproportionate number of chicks that pipped in the middle of the egg or the narrow end. Some of them got out OK, but about 4 of them I helped by opening a small hole in the shell so they could at least breath until the got out. Two needed extra help on top of that as they were malpositioned and could not get out of the shell. It is important to note that intervening too soon should be avoided since the chick needs to absorb the yolk before leaving the egg.
One thing that annoys me about my incubator is that the chicks that hatch early run around and knock into the eggs that have not hatched. This sends the eggs rolling around and and I don't think it does any favors to the chicks that are trying to hatch. I removed hatched chicks every 6 hours or so. When I did it, I sprayed the sides of the incubator with warm water from a squirt bottle. I never had problems with the humidity going too low.
All 20 of the eggs that made it to lockdown hatched by day 22.