Man of Honour
Maybe I'm muddling things up. Either way, the front brake is way more effective in actually slowing you down, so should be the main one you use.
The proof of this is just to push the bike and brake. On all 4 of my bikes if I front brake I can't push it any further without the back wheel lifting off the floor. If I back brake I can still push the bike with the back wheel sliding across the floor.