Crushing just before mashing is best from my experience. If you crush the grain and let it sit for more than a few days or so the fatty acids in the germ, which you expose to air on grinding, begin to oxidize. This can result in some off flavors in your beer.
The rest period I think you are referring to is for freshly kilned or dried malt as others have referred to here. The theory there is that in commercial processing at least, the malt coming off a large kiln has a range of moisture from say 6% to 3%. Letting it rest in a silo allows the moisture to equalize to the normla range of 4%. This is important to commercial brewers as the moisture of the malt affects how it mills in terms of particle size distribution.