I've never waited till the first pip. I'd say if the humidity was too high for much of incubation, then the air cell will not be large enough and they could drown when they pip into too small a space or no space, but I have great hatches by raising the humidity to 65% or so anytime on Day 18 or 19.
Good Luck with them. Last hatch I had the all hatched on day 22 & 23. I guess my hova was a little low on temps so I flipped the switch to the next one up. So will see I'm using it to hatch out some frizzles so I'll see how it goes this time around. I have the hova bator 1588 that is pre-set but it must have been off.