Beausol, thanks for the detailed explanation. I think the reason most kits and DIY wings use the vertical grain is just for the ease in cutting them and most economical use of the wood. Since the stregth is still usually adequate (definately an improvement over no webbing) either way will be a plus, it is just that, technically, grain oriented at 45 degrees results in the strongest web for forces in one direction (weakest in the other) while either vertical or horizontal grain is a compromise that works and helps in both directions of force.