I assume by bad back, you mean you have some degree of pain, soreness, stiffness or discomfort in some part of your back. The secret is to learn to listen to your body and know when the problem is just something that means you had a last good workout or is something that requires rest or medical intervention (like any other pain, soreness, discomfortor stiffness in any other part of your body). That is, is the pain in the muscle or the joints and connective tissue? If the pain is in the muscle, and you are not grossly overtraining, then more exercise will not hurt anything and might even help. However, if the problem is in the joint or connective tissue then more exercise might make it worse.
One thing that might give some evidence would be to stretch or do some very light weight, full ROM, high repetition movements. If the pain tends to go away after this then is might only mean you had a good last workout. If the pain gets worse after this is might mean that you need rest or medical intervention.
Of course, the best course of action, if you don't know, is to rest and see your doctor if it continues.