You don't have to take any more chances than you feel comfortable with. If you're concerned there might be boogers in the rock, hyper salinize it. If you find brittle sea stars, amphipods, and other goodies, scoop them up and drop them in your tank. If a 3' fireworm crawls out... RUN!!! LOL... just kidding. You'll want to remove things like mantis shrimp, and bristleworms, unless you don't mind britstleworms. Most people consider them beneficial to the ecosystem. I don't worry about them in my tank, unless I pick up a rock. They carry a nasty sting, or so I'm told. You'll have to physically inspect the rock for evidence of aiptasia or mojano. These anemones are invasive, and can be hard to control unless caught early. Small cases can be blasted off the rock with hot water and a turkey baster. There are commercial types of eliminators that have to be fed directly into the mouths, but as previously mentioned, they can easily be eradicated if caught early. A lot of it will depend on where you buy the rocks. The only LFS within 100 miles of me is "where the pets go", and I'd have to boil the rocks that they sell. They're horrible! The rocks that aren't covered in aiptasia are covered in hair algae. If you have a good selection to choose from, just look for the cleanest ones possible. You can add growth (ie: corals) later...