Quickly opening the incubator shouldn't hurt if you feel the need to do a quick candle or add a wet sponge to raise humidity. Shrink wrap doesn't usually happen quickly, it has more to do with conditions throughout incubation or prolonged periods of low humidity after external pips. I've heard of some breeders not even raising the humidity or putting chicks into lockdown until after they see a first internal pip or even in some cases an external pip (I wouldn't want to wait for an external personally but it goes to show eggs are not as affected by low humidity early on in the hatching process as some think). I think as long as you keep your humidity above 60 you should be alright, I like to have it a bit higher if possible in case I need to assist and have to open and close the incubator a lot.