This is an old argument that always gathers much comment. If you check out your Strength and Materials handbook, you will see that the stresses in any web is at a 45 degree angle to the span so it really makes no difference in whether the grain is vertical or horizontal in the webbing, both are equally strong in shear loads. Now, if you are worried about crushing (say the cat or dog steps on your wing) then vertical grain is the strongest. If you want maximum shear strength, you need the grain at 45 Degrees to the span but; if you go inverted the shear stresses reverse by 90 degrees so now that same 45 degree is the weakest direction. That is why the specifications for shear webbing on homebuilt planes usually calls out plywood with the grain at a 45 degree angle to the span to give maximum shear strength for both positive and negative G loads. In our models, don't worry, either spanwise grain or vertical grain is equally effective.