IMO, it doesn't hurt them since the hen would get off of the nest. I don't purposely do it, but I don't fret if they're out for a while. It doesn't change the length of incubation time. I cooled one batch of duck eggs and then the next batch didn't cool at all, the majority hatched on the same day. I think it's unnecessary, but it definitely doesn't hurt. I have heard that cooling eggs with small air cells, or cooling shipped eggs might help because it helps them achieve better weight loss, but I've never tried it because my air cells are usually fine. If I were you I would probably not do it unless my air cells were on the smaller side, but my incubator doesn't have a cooling feature so it's more work for me to cool them. Let us know what you do, and how it works out!