The shoulders are a very complicated joint and I am no expert. Your problems may be different from mine, so I am not sure what works for me will work for you. In my case, it was my range of motion which was limited, in part because of an old injury. Keeping my shoulders back seemed to increase my range of motion. In a recent post, someone spoke about "hollow body exercises" in gymnastics and pointed out to me that the trick was not to keep your shoulders back but rather down. I discovered that this made a big difference for me. All of this helps to improve my posture and how motion transmits between upper and lower body, but it also helps my shoulder range of motion. That's for motion out of the pool. In the pool things are more complicated because the shoulders are moving forward and backward, but I do have the feeling that having good posture in the pool also increases my shoulders' range of motion and the comfort I have moving my shoulders.