It depends on what machine your talking about. I usually try to use free weights as much as possible, but when I do use machines, I try to keep my grip as far out as possible. Although it does depend on the exercise and machine your talking about, basically the further your grip is the more of your outer back your working and vice versa. If you are only using machines, then it is a good idea to switch back and forth to ensure that yuor not cheating any part of your back.