Well the best rock in my experience is from Tampa Bay Saltwater. However, all rock now is aquaculture rock no rock is actually Fiji rock its illegal. With that said yes you can get bad hitchhikers but good ones also. Depends really on the size of rock you're getting. The bigger size may have mantis shrimp, I doubt though your LFS will have that issue. I have never seen better rock than Tampa Bay though anywhere.
Tampa Bay depends on the year you get it, I have got a lot of Gorilla crabs, I also got a octopus (gave him to my LFS). I got nano size rock and didn't get a mantis. Keep in mind bigger size rock and more rock the more likely to get some pests.