I back in at work, even if I don't have to every now and then to practice. I almost always back in at malls and "real" lots with people because it's easier and safer. You back in, you might hit some retard jumping into your spot. To avoid this, just look back and keep scanning.
I find that if I back out, I have to concentrate too much on where my ass is, where the front end is as I steer, and traffic in a live lane that I can't see well, including peddies. So, gotta focus on 4 things. This doesn't even include the fact that you can't see shit until half your car is in a live lane, and it's hard to watch both sides when your straining to look behind you.