The question of maximum principle stress angle is a good one. I opened up my mechanics of material book and the example problem for wide flange beams shows to assume the shear stress parallel with the beam and normal stress perpendicular. I didn't take the deep dive on the fundamentals, so this is just a regurgitation of printed material. The example they gave to visualize why was a leaf spring. Not too much help really.

The nature of the material (anisotropic) is also key, as banktoturn points out. Balsa has very low shear and high tensile strength parallel to the grain. The opposite is true perpendicular to the grain. Ideally, you want the shear and normal stresses aligned accordingly. As far as my text goes, that would be vertical in a wide flange beam.