While one might expect this to be true, in my experience of wearing ballcaps and moving at above-walking speeds it's actually the forward orientation that holds best. A backwards bill often presents a larger surface to the wind, especially if it's pulled low against your neck. Moreover, the forward orientation is kind of "hooked" around your forehead, whereas the backwards orientation basically is just sitting on the cap of your skull, so less wind force is needed to unseat it. Forward caps the wind needs to kind of peel it off, backwards orientation the wind just pushes upwards lightly.
