My live rock was something of a special case, as it was cured, but had been living in a gargantuan tank with other live rock for a LONG time...it had many odd critters living in it. In fact, the seller specifically picked out a few crustaceans...not sure if she wanted to keep them as valuable or something, or what, but here explanation was "I don't know exactly what these are, so you don't want them." I guess on the thinking that they might eat something I don't want them to. Given that I was building an octo-tank, I don't think that was an issue, but maybe she was just trying to protect me.
Anyway...I think my cured rock was just great, but I'd listen to the real experts!
rusty