I agree that it makes little difference in our models on which direction the grain goes. It just goes against my nature to not comment on the errors in some of the above statements. Find yourself a good mechanical engineer or good engineering text book and you will see that (when a beam is under a bending moment) the shear forces in the web are at a 45 degree angle. True, if you step on a wing laying on a flat surface, a vertical grain will be stronger; but, that is not what happens in flight. In flight, you create a bending moment, not direct compression. As you go from upright flight to inverted flight, the direction of sheer forces changes 90 degrees. However, in upright flight the stresses in the upper spar are span wise compression loads and in the lower spar tension loads. These also reverse for inverted flight. The maximum stresses in the spar is on the very outside surfaces so thicking the spars does little to improve strength although widening them does help in direct proportion to the width increase.