On the bag I'd more or less aim for the same spot. However, put it in context. If you're doing shadow and you imagine your opponent's actions in response to what you throw you might decide that his body is open after jabbing to his face and do the cross to the body. You aim for your target wherever that is. If you find it difficult to balance doing that then the trainers might have some more suggestions for you which I won't try to answer in case I stuff up.