If you research shear webs, you will find that the strongest are those with the grain at 45 degrees to the vertical; bad thing, this changes by 90 degrees if you switch from positive G's to negative G's. Short story is that you are equally good having shear webs at either vertical grain or spanwise grain as far as shear strength goes. Now, if you are worried about crushing the wing by stepping on it or the dog walking on it, then having the grain vertical is the strongest for that external force. Shear forces are always at a 45 degree angle to the vertical so a pure shear web, for maximum strength with minimum material, the grain must be at 45 degrees to the vertical. Again, the bad thing is if you set it up for maximum resistance to shear for positive G's, you will have minimum strength for negative G's. Either vertical or span wise grain is the compromise for best strength for both positive and negative G's.