The gen rolex case opener is indeed a useful tool for it.
...all the Rolex reps have screw-down casebacks, and if they are closed properly you won´t be able to open them easily.
The thing is, some are just screwed with their hands, so the "rubber ball" trick will open then, but It´s not a 100% rule. Get any tool for screwed waterproof casebacks on the bay and you´re good to go..
The gen rolex case opener is good to open the case very easily and making NO-scratches on it, as it fits perfectly on the caseback, not letting the tool slip and scratch the watch while being opened or closed.
Cheers,
Spirit.