A couple decades ago, when I first began quilting, popular wisdom was, always on the bias. The reasoning behind it, (aside from selling more fabric) was more threads on the bias, hence, stronger. However, if using good, quilt shop quality fabric, and folding in half when sewing it down, there's more than enough threads to ensure good, even wear.
The only time I use bias binding, is on scalloped edge quilts, or if the binding fabric used would look better cut on the bias and I have plenty of fabric to do it with.