Chicks can go up to 72 hours without food or water, as the final bit of the yolk that they absorb right before hatching out gives them enough nutrition to tide them over for a couple of days. So if you have some chicks hatched out already and some eggs just starting to pip, you can leave your bator shut, keep your humidity up high, and not worry about the little guys starving. They'll be perfectly okay in there for a day or so. In that situation the most important thing is to observe 'lockdown' and keep the bator shut so as not to harm the chicks that are still to hatch.
Of course, if everyone hatches at once and fluffs up quickly then that's your hatch finished and you have no reason to keep them in the bator any longer than necessary, so in that case you can take them out and transfer them to the brooder with food and water as soon as you like.
Some people like to reduce the temperature by a degree or two once the eggs are in lockdown, or right after they hatch, but it's really just a matter of preference whether you choose to do it or not. The thinking behind it is that as the chicks arealmost fully developed by day 19 or 20, they no longer need the same amount of heat. After all, once they hatch you'll be transferring them to a brooder where the absolute max temp is 95F. So I suppose people think that a slightly lower temp will be more comfortable for the little guys once they've hatched.
Personally, I don't fiddle with the temp control till everybody's hatched and dried off and I'm ready to switch off the bator. My thinking is, if it's sitting steady, don't mess with it. I've never had a problem with new chicks being overheated or getting dehydrated, and somethimes I've had to leave them in there for nearly the full 72 hours. But if I don't have to do it, I like to get them in the brooder as soon as I can. There's no reason to leave them in the incubator for precisely 24 hours after they've hatched. If they fluff up and get on their feet in six hours, that's good enough for me.
What I will sometimes do though is transfer the older, stronger and more boisterous chicks to the brooder and leave the younger and weaker looking ones in the bator for an extra few hours, half a day, whatever, to give them some peace and let them find their feet without being trampled all over by the ones that hatched out first. Sometimes some of them just look like they need an extra few hours to lie out flat sleeping and getting their energy up...
