Because of the tendency we have to use our anterior muscles (chest and front delts) more than our posterior muscles (especially upper back and posterior delts) in everyday life, and to sit at desks and in cars, many people have anteorior muscles that are too tight and posterior muscles that are less strong. It's important to avoid overworking the chest and anterior shoulder during workouts. In fact, if you already have a visible imbalance (chest/shoulder and possibly abs too tight, rear muscles too loose), I'd recommend doing more back work than chest until things balance out.
Many workout DVD's seem to overwork the 'mirror' muscles (anterior) to the detriment of the muscles in the posterior chain. Even Cathe's earlier workouts do this (not necessarily in number of exercises, but sometimes in weight choice, like when Cathe does bench presses with 25# dumbbells, and rows with 15#).
I'd recommend making sure to do 1-2 sets more for back than for chest, choosing similar weights for both, and doing exercises for the rhomboids and upper/inner back (like horizontal rows, rear shoulder flyes, etc.). And make sure to stretch the chest and anterior delts regularly. Also, IMO, anterior delt raises are not necessary, as the anterior delt gets well worked during chest presses and push-ups. My posture--which was already pretty good--improved when I stopped doing most isolated anterior delt work and made sure do to rear shoulder flyes.
Balancing out the muscles around the shoulder (chest vs. back, anterior delts vs rear delts) is important for more than just esthetics: unbalances muscles around the shoulder girdle can lead to rotator cuff problems.
