Typically I will cut somewhere within the last 2 weeks of incubation, my incubator temps are steady around 89.5 F. Occasionally if I have an egg dimple very bad and I cant get it to pop back as in a clutch I just hatched I will cut a little earlier but will wait until as late in the incubation as I feel I can . Umbilicus probs do concern me more than a neonate not having an egg tooth but, umbilicus probs can still happen after cutting.