I totally disagree about the grain direction. It should be veritcal, no question about it.

The biggest loads on a wing are in bending. Both inverted and upright. When the wing bends, the spars will tend to try to get closer together, creating a compression load between them. It can be enough to cause ribs to actually crush. I've had wing spars in foam wings crush all the foam between the spars and "come together" a couple of times as well. To handle that kind of load, you need the grain vertical.

Sure, there are other loads involved, but by far the biggest, and the one you really need the webs for is that compression when the wing tries to bend.