Good stuff. The reason I mentioned external rotation only is that most exercises involving major muscles involve internal rotation.
People tend to think that pec major and latissimus dorsi balance each other out...actually, they both internally rotate the shoulder.
Internal Rotators:
Pec major
Anterior deltoid
Lats
Teres major
Subscapularis (rotator cuff)
External Rotators:
Posterior deltoid
Infraspinatus (rotator cuff)
Teres minor (rotator cuff)
Probably forgetting some...but anyways when most people work out, they tend to develop the first group more than the second.
A decent measure of muscle balance is to relax your arms to your sides, and look where your elbow "pits" point toward. If they point inwards instead of directly forward, your shoulders are internally rotated.