I use to believe to start skimming when I added uncured liverock to my tank, now Im reading it really doesnt matter, skim when you are cycling or skim when your done cycling, it seems from what I've been reading that it makes no difference.
I'd recommend skimming right from day 1. If you have a tank full of uncured rock and you're not skimming, ammonia levels could easily get excessive during the curing process.