My 2 cents....which is consistent with the wedge and msafi65....

The chevron pattern is designed to be a self cleaning tread (I have plowed many a thousand acres in reliance on chevron tread tires). If you think of the chevrons as arrows, and point the arrows on the tire in the direction of rotation, then the treads will dig a hole and clean themselves (to a certain extent, depending on soil type and wetness) - good traction on the ground, but bad traction in the water. Turn the tire around and you get the opposite effect. I run all 6 of my tires backwards, meaning the arrow points in reverse rotation because 1) I don't want to dig my hole any deeper, and 2) because of my low ground clearance I prefer that little extra bit of traction in hopes of being able to back out of a bad spot.