This isn't always the case. Depending on the quality of the bike/fork...even with good quality bikes/forks. In some cases one of the dropouts might be welded/brazed/glued in just a fraction higher/lower than its mate. Or, one side of the fork could be a fraction short/long. In either case that fraction could result in the outer edge of the wheel to be noticably offset. Therefore, I think it's better to insert the wheel, and with the quick release open...hold the wheel centered in the fork and then close the quick release. Sheldon Brown says that the quick release should just be tight enough that the handle makes a slight impression on your hand when you close it. If the wheel doesn't spin freely because it's too tight...open the quick release and back the nut off about a half-turn each time until it does spin freely when closed.

Dan