when i show up at a bar sober and i back into my parking spot i am assuming that risk when i am sober and i am more comfortable with it then. When i leave the bar and i am hammered drunk i don't want to have to fuss with backing out of a space and try not to hit other cars when im drunk. so if you back in sober, then you can just pull straight out when you leave drunk.
On a serious note though, its way safer if you back in so when you are ready to leave its easier to get out. Any parking garage down town will prove this when a concert/sporting event lets out.