Dear B. Challenged:  Breathing on both sides (bilateral breathing) is very important in developing a symmetrical stroke.  If you always breathe on the same side, you are likely to develop shoulder/neck problems from the asymmetry.  (I can vouch for this through personal experience!)  During your training, especially warm up and warm down, you want to practice breathing on both sides.  If you find it difficult to breathe alternately on both sides, you can try 25m breathing only on your right and then 25m breathing only on your left.  Spending some time breathing only on your less comfortable side is also a good drill.  That said, bilateral breathing is more important in training than in racing (as you generally spend much more time training).  Coach Ernest Maglischo, author of "Swimming Fast", "Swimming Faster", and "Swimming Fastest", has said that breathing every two strokes (i.e. every stroke cycle) while racing works best for some distance swimmers, as the need for oxygen is more important than symmetry.  However, for those of you who race in open water, the side you breathe on may be dictated by the waves.