Originally Posted by Modshack

Couple of things actually. First, assuming your alignment is spot on from the factory Makes you a very optimistic guy.. In reality they have but a few minutes on the assembly line to get all 4 wheels adjusted. Sometimes they do, sometimes they don't. My New Corvette had 3 of the 4 wheels out of spec.

Secondly, most lower suspension arms are bolted to subframes, which are in turn bolted to the chassis. Taking a hit to a wheel can shift the subframe slightly therefore throwing the car out of alignment.
A shifted subframe will take 2 wheels out of alignment, not just the one that took the hit.

Consider Alignments a normal maintenance item. Good idea to get one done when new by a quality shop who specializes in alignments, then get it tweaked once a year if you live in an area plagued by bad roads. The money you save on tires will generally pay for the alignments ....