Out of practicality vertical grain proves to be the best (45º two ply is the ultimate but a pain to make.)
Shear forces always exist at 45º to the direction of the applied force, so you could imagine that either of the 90º variations of vertical or horizontal would equally suffice, but, as it was pointed out to me a while ago, NON inflight force direction is always vertical and so the grain should be.
Imagine an "Oops moment, I just dropped, lent on, packed something on top of (etc) on the wing." Now, which grain direction would you prefer here to avoid crush damage?
But, after thinking about the inflight forces logically I think its more about the aspect ratio of the bay that the webbing occupies.
All wing bays are 'landscape' in orientation, in that they are longer horizontally than vertically and so its better to strengthen between the two shortest points, and that's vertical.
Now picture a fuselage bay being 'portrait' and now the two shortest points are horizontal.
Picture two parallel engine bearers, the two shortest points are ....... well I am sure that you get the picture.
I would rather run the grain between the two shortest points than not every time.
Just to cover this off again, the two 'shortest' points in shear are always at 45º and wood is less compressible along the grain - so think simply join the two shortest points!
Hope that helps.