If your eggs are almost fully developed to the point where they are now pipping and getting ready to hatch, a slightly low temp is not going to do them ANY harm now. Think about it. A few more hours, maybe a day, and they'll be hatched, and then you're going to put them in a brooder that's 90-95F, and immediately you'll be worrying about overheating them. Your dead chick was almost certainly not due to your slight temperature drop.