If you're waiting for the rock to cure before moving everything into the 75, then curing it for a little bit is a good idea. If you're moving everything to the 75 and the 75 is running for a couple weeks, then I wouldn't bother curing it, I would just put it all in at the same time. Reason being that even if you move everything from the 55 to the 75, you're going to have a small cycle, just because everything is stirred up. All of the benthic organisms that are buried in your sand now will be introduced back into the water column and that's bound to cycle you a bit. I'd rather have one larger cycle than draw out smaller cycles over a period of a few weeks.
Eric