Where you place your foot on the rock step should depend on what hold you are using. If you have a double hand hold, it would make sense to take the rock step straight back, as you have the ability to extend the hold a little bit to accommodate the increase of space between partners. However, if you are dancing in closed position, your ability to extend the dance position becomes pretty limited, especially on the "closed" side (the leader's right side and follower's left, where the leader is holding the follower's back), in which case it would make sense to hook the rock step a little bit. Obviously, over crossing can create its own set of issues, which is why a 5th position is a good approximation. It will give you just enough room to take an actual rock step, but it's not so crossed that it should twist your hold out of position.
As with most techniques, the trick is to understand what that particular technique causes and then add new techniques as the circumstances of your dance changes. I hope that's helpful!