I would say it's not NECESSARY, but it's very BENEFICIAL, especially if you are doing heavy pressing work. It won't do much for your pressing but it can do a LOT for injury prevention.
My RC work usually involves a YTWL-based shoulder warmup (usually with 15 lb dumbbells), and then after my shoulder workouts I'll do 2 sets of 12 on internal and external cable rotations. Some have suggested that it's better to do RC-specific stuff at the beginning of the workout.
Basically when I train my RC I do it at relatively high reps and low weights. Keep in mind the rotator cuff consists of smaller muscles, so you don't want to go very heavy on them.