I only commute with one pannier. At first I used my right pannier, but I switched to the left, not so much because of the widening of the bike or any other good reason, but I end up swinging my right leg off the bike and end up on the left side after I arrive at work, so I figured it's more convenient to have my pannier on the same side I am.

Plus the way I store my bike at home is much more conducive to using the left pannier.