When you're strength training, there are definitely certain multitasking moves where you sit, kneel, or lie on the floor or a balance ball. I think that is fine, but for all other upper body exercises, be sure to do them standing instead of sitting on a weight bench. Standing makes the exercise more challenging to your entire body. Plus, standing helps ensure that you are working with proper posture and not hunching over. This is better for your spine and works your core too.