Best bet is buying cured live rock from your LFS
. That way whatever is on the rock is pretty much guaranteed to stick around.
As far as uncured goes, (I only cured once because it was a hassle, but WOW was there a lot of life on that rock) Timora live rock is the best. It's really porous and just covered in bits of sponges, corals, dusters, etc. Most of the sponges are going to die curing as well as anything already in the process of dying.
But yea. If you are buying uncured you HAVE to cure it before placing it in the tank. Quarantining after it is cured is pretty important too.