To throw an effective punch you do have to be loose. If you're all tense and tight when you throw the punch than the punch will be slow and ineffective. You've got to relax when throwing the punch, experience in the martial arts should teach you that. However, you do want to be tight at the moment of impact. If you're not your wrist could bend when you make impact which you don't want. I used to have that problem myself. Anyway, what I've been taught is that at the moment of impact you want your hand to be as hard as a rock but the rest of your body loose. By making your hand as hard as a rock at the moment of impact you will avoid bending your wrist but you want to try to keep the rest of your arm and the rest of your body loose. Hand strengthening exercises can help with this. Prior to impact you want your hand loose but at the moment of impact you want to make your hand hard while keeping the rest of your body loose and then to quickly retract it.