Working the opposing muscle groups isn't so much to prevent tendonitis, but to keep the range of motion and joint stabilization. If you've ever seen someone who only does bench presses and/or push-ups without doing rowing type exercises you may notice them having a rounded shoulder posture. Have you seen some of the football or basketball players on "Dancing With The Stars" try to assume a proper posture in hold? It's funny to see, but they're so used to being in a crouched position it's hard for them to stand fully upright with their shoulders and head back because, to them, it just isn't natural. Believe me, we worked folks pretty diligently on this in Physical Therapy.