If a beam is loaded such that it is hogging (supported at each end with a downward load in the center) then the upper flange will resist compression and the lower flange will resist tension. The web which is what we are discussing here will resist shear loads. It can be shown that there are both horizontal and vertical shear loads, to resist horizontal shear you would need the grain to be running vertically and to resist vertical shear you would need the grain to run horizontally. Most engineers rarely design in wood so it is never an issue for them but based on the preceeding I would say that ply would be the best solution.