While they may be a concern, I certainly wouldn't avoid ordering TBS rock simply because of them.
They key is to keep a close watch very early on, before your tank is filled with corals and whatnot. When you spot a mantis or gorilla crab in a rock, just pull the rock out and use ro/di or carbonated water to eject him. I personally removed about 6 mantis and countless gorilla crabs from my 100 pound package. The only ones that actually caused problems were a pair of gorilla crabs that had grown VERY large. One clipped the leg from a serpent star, and another gashed open a star's disk. (Both stars survived and are still with me. The gorillas weren't so lucky.)
Don't worry about it. Yes, they can be pests, but look at capturing them as a sport. It gives you something to focus on while the tank cycles.