I wouldn't recommend using locking casters on any bench. They still allow the bench to move. Instead, use casters that can be raised for using the bench and lowered to lift the bench for moving. I had some like this on a table saw. I got these at WoodCraft.
Like Dave, I also have a 400 lb joiners bench, but I still have to shove it up against a wall to keep it from moving when I am jointing a board. I think anything out in the middle of the room is going to have to be secured to the floor somehow. Of course there are many ways to do this. If you have a wood floor, then a metal bracket and some lag screws will do. If you have a concrete floor, then you will need concrete anchors and lag or machine screws depending on the anchors.