Originally Posted by
Dan Burkhart
Actually there is a structural reason for this. Opposite the valve hole is the rim seam, the place where the hoop is joined by pins or welding. It doesn't matter too much with a welded seam rim, but if your rim is pinned, the spokes that cross it are supposed to draw the sides together. If the spokes cross at the valve hole, they will be parrallel at the seam.
Doesn't matter. All the spokes are tensioned and pulling the rim in toward the hub. That puts lots of compressive force on the entire circumference of the rim - there's no way that a seam in the rim is going to come apart regardless of where it is located.