Rich,

I agree that the strength when supporting spars the grain should run horizontal to the spars. But I think that in most model aircraft the main weakness of the wing is not in the tension and compression of the spars, but in the compression of the weakest point in most wing builds which are the ribs. Shear webs on most every model that I have built or reviewed, they are always perpendicular to the spars. Most ribs are 1/32nd and shear webs on the first 4-5 rib sections. I am not an engineer, but the area of highest structural stress is on the wings near the fuselage. I think that 1/4 or 3/8 spruce spars transfer this stress along their length and transfer a large amount of that stress to the ribs. The shear web is designed to transfer loads evenly from both spars and to keep the ribs from bending or crushing and therefore if unidirectional, then they should be in compression between top and bottom spars with grain perpendicular to them.

JM2C