I think it is necessary if you have any shoulder issues. If you don't and your form on lifts is good, then you might not need to. I'd say if you're worried about it, you should give it a go.
A good test is to take a 5 pound plate, hold it out from the shoulder to the side, upper arm parallel to the floor, elbow bent 90 degrees up so lower arm is straight up and down.
Now, rotate the weight downward keeping your elbow in exactly the same place. All you're doing is rotating the plate around your upper arm so it goes from pointing straight up to pointing straight down. Then rotate it back. Do the other arm too.
If you can do that with no pain or weirdness, weakness, twinges, etc. then your shoulders are fine.
If you can't then I'd do the rotator cuff stuff. I'm by no means an expert, but I had an impingement last year that took weeks of physical therapy, so I'll tell you what worked for me.
In therapy, and still to this day, I do interior and exterior cables like Aurik said, I also do band pulls, which is the opposite of the plate movement above, where your arm is out and you're rotating the band up. And the plate throws too. I also do what's called a "falling star" where you lay sideways on the benchwith your top elbow in your side and a dumbell 90 degrees forward in front of you. Now, rotate it up like an exterior rotation, then when it's straight up, press it to a straight arm, then lower the straight arm until the dumbell is stright out in front of you. Pull your elbow back into your side and that's one rep. Works the whole shoulder assembly, and it has helped me a lot. As Aurik said, do light weights, 12-15 reps.
I do 2 sets of 15 of 2-3 of those exercises. I started with 5 pound dumbells and the lowest settings on the cable, and raised weights as it got easier.
Also, straight arm cable pulldowns, front and especially side, are good for the shoulder too. You can do much more weight with those though, as those work the bigger muscles around the shoulder. That wasn't my problem, so I don't do those anymore.
I do rotator stuff AFTER my normal workouts (3 times a week) when my shoulder is already as warm as it will get. I think stressing a cold shoulder is a bad idea. Arm swings, dislocations, light stretching, empty bar work, sure, but rotator stuff with weights and then going into a heavy workout? Not a good idea in my therapists' opinion.
I also learned some stretches, especially a "sleeper" stretch that is enormously helpful (hard to describe, look it up), and I do those stretches, especially that one everyday.
Might be too much information, but I guess if you slogged through it, it must have been helpful. I hope so!
Last edited by MikeM; 04-29-2011 at 01:16 PM.