That's why you should vary sitting and standing, and not sit for long periods. If your back is being supported, then it is not strengthening the muscles that are supposed to support it. Which is why swiss balls are good; they make your body work to stabilize itself, strengthening muscular support of the skeletal structure. But, like I said, if you are aware of your body and can engage muscles and vary posture even when you are in a comfy chair, then I don't think it's a huge problem.
