I was taught to always back-in to a parking spot when possible. Reason being, backing up (now or later) takes slightly longer, and while you have the extra ~30 seconds now, you may not have the luxury of that extra time when you need to leave (e.g. in some sort of evacuation order). So, spend the time now to save yourself later.

Edit: I was taught this by my dad who did a lot of work at chemical plants and oil refineries. Probably made more of a difference for him than for a software engineer like myself. Regardless, the principle still applies.