When I was a kid learning to drive, I remember using the hood ornament as a "crosshair". After I got my license and my own car (w/o a center reference), I haven't needed any kind of center point. After driving a particular car for awhile, it usually becomes an extension of my body and I don't need any reference points to know where it's corners are, it just seems to be instinctive. I back into my garage every time, and I park really close to several obstacles, yet I seem to maintain a repeatable precision.
Unless I'm in a hurry to arrive somewhere, it seems logical to back in when there's an option. The reasoning is that I might just be in a big hurry to leave on time, (Earthquake, Fire, Act-of-God, etc.) so being able to pull right out is nice. I'm usually not in any hurry to park, so I have time to back in. I think it's all this backing over the years that has made me good at estimating where the car is.