I've worked for a few companies with policy that anytime we were driving company vehicles, or driving our own vehicles on company property or on official business, we were always supposed to back in to a parking spot. The reasoning presented during the painfully boring orientation was always explained thus:

Backing in means driving past the spot first, allowing you check that it is free of obstacles such as errant shopping carts or stray children. Pulling out forwards gives you better visibility, both in terms of those pesky children who might be walking or crawling past the spot just as you try to leave, and because you can see oncoming traffic better.

Personally for me it depends on the situation. For shopping I usually front-in so that the trunk is accessible to stow my cargo. Then I just give the shopping cart a quick shove in the downhill direction and I'm good to go. In confined spaces like parking garages sometimes backing in is easier because of the tighter turning radius. When cruising for a spot in a crowded lot, if you intend to back in then you don't have to keep slowing down when you think you see a spot but it's always just a tiny car. Instead you can fly up and down the lanes at an unsafe speed, and when you pass a truly empty spot just slam on the brakes and throw it in reverse. In the winter if I think there is any chance I might have to shovel later, I back in to make it easier to just plow my way out instead of shoveling (then stop to pick up that plastic piece and throw it in the back seat). On the other hand, if the street is slippery, fronting-in always gives an excuse to pull a 90 degree handbrake turn, so it depends.