I grow rugosas and would still prune if it were mine. They don't seem to mind when they are pruned.

Have an idea in your head of what the final goal of the pruning is going to be. Old and dead wood should be removed at the base. It is possible that is all that should be done. If the rose is getting too big for the space, then do some cutting back. Some rugosas have attractive, fountain shapes, and can lose that is they are butchered, which is why having a goal is important.