Surely the layered construction (as James points out) of the plywood itself is where the intrinsic strength is? (I'm no engineer though...). However, could it be the case that the early understanding of aerodynamics indicated that the airflow over the fuse was 'cleaner' when the grain ran in the direction of airflow rather than at angles? I know that the ply was not bare as such.
Also, as evidenced by the awesome builds accumulating here, modellers vary the direction of the grain and I take it that that is historically correct, yes?? (which would negate the 'clean' aerodynamic question...)
Andrew