Well, it's beyond just shoulder alignment during the exercise. Your shoulder and upper back posture is pretty much dependent on the interplay between your pects

Well, it's beyond just shoulder alignment during the exercise. Your shoulder and upper back posture is pretty much dependent on the interplay between your pects (chest), upper and lower trap, and lat muscles. If you start getting a severe strength imbalance between these muscle groups, you can start getting posture problems. That's in the long term. There's also short term stuff with your sore muscles avoiding stretching, resulting in some curling in. All of this only gets worse if your typing at a keyboard because of the 'natural' position you assume.