I always back into parking spaces. For me, it is also a safety issue. It is easier to see backing into a space than backing out. Also, if your battery dies or something else happens where you need to get under the hood, it is much easier to do this when you have backed into a parking space. Trying to get or give a jump when a person has pulled into a space is more difficult (and I have, unfortunately, been in the position of having a battery die or car not start when I have been parked in a parking lot).
99% of the time, I can back into a parking space correctly the first time, without all of the backup, pull forward, backup, pull forward, backup business...
One of MY pet peeves is people who blow thier horns at people who are backing into a parking space, as if it's somehow illegal or wrong.

If the few seconds it takes to wait for someone to back into a space is going to make you late, then you should really consider leaving your point of origin earlier.