My vote goes for backing in. Two reasons: First, as already mentioned by uitlander, when backing out there are many more things that you can possibly hit. Second, and I'm surprised no one mentioned this, is that when you are going head first into a stall, the steering wheels are in the front, so the rest of the car has to follow the line that is set by the front. That's why so many cars have dings and scrapes on the corners of their bumpers, from other drivers who cut the turn too tightly when pulling head first into a stall.
When backing in, the steering wheels are in the back, so the back of the car can be placed, and the line the rest of the car takes can be adjusted, much more so then when going in head first.