When to Move Queen Cells from a Cell Bar?

I most often move mine soon after they're capped, but I've even placed them soon after the grafts were accepted but well before they are capped.

I have read that the cells (larva) are fragile for the first couple of days after capping. I know that I have found dead larva lying in the bottom of unhatched cells before. I used to pull a frame and after finding cells flip it over looking for other cells. After reading about fragile cells, I thought that maybe the reason that the cells didn't hatch was my handling of the frames too soon after capping.

They are very fragile right after they are capped. Plus they are very succeptable to cold and heat. A mating nuc is usually not well equipped to keep the queen cells as warm and cool as a full sized hive.

Another reason is because of the lost brood rearing time by making up the nucs early. By waiting until the cells are ready to emerge, you get another weeks worth of brood rearing from the hives you make up your nucs from.