I've only seen one guy do it at an indoor range. I admit it looked a bit silly to me, but I knew what he was doing. He also did that move where he pulled the gun close to his body before looking both ways.
I suppose it becomes a state of mind, similar to how we never point a gun at anything we don't want to destroy. Nor do we break the other three rules. If I'm talking to my kids about a gun, and hold a pencil pretending it's a gun, I don't point it at them. They think it's silly too, but in my mind it's a gun. I try to ingrain that in their heads too.
I might start doing the lookback move too, but just at home for now.
Regret for the things we did can be tempered by time; it is regret for the things we did not do that is inconsolable."
-Sydney J. Harris