Alignments are hit or miss. It's the one repair that you may have to get redone. It seems that techs don't take care to center and lock down the steering wheel - they seem to forget that once they start adjusting the suspension parts, the steering wheel may move around since the steering mechanism is tugging on it, and the car's own steering lock has a significant degree of play.
That said, an alignment isn't something you should have to get frequently. A hit that is hard enough to affect the alignment is pretty severe and would likely have also damaged a wheel.