Bill,
First, this is my observation, I am open to correction by someone who knows better, but this is what I have learned and experimented with so it is my own understanding.Your mileage may vary
the strentgh of wood lies along its grain. To maximize the use of its inherant strength you want the grain to run in the same direction as the stress. for example, when you twist a wing it wants to flex diagonaly from the trailing edge to the leading edge. If you orient the grain of the wood so that it lies parallel to the motion of twisting, (the axis or rotation) it will more efficiently resist the torsional movement. by aligning the ribs at an angle to the leading and trailing edges it more accuratly aligns the strength (grain) of the wood with the twisting motion in the wing. The leading edge sheeting resists the bending of the tips up and down because the grain is aligned with the direction of motion, or axis of rotation. In a conventional wing configuration the ribs and sheeting are perpendicular to each other and have comparativly limited resistance to torsion, twisting. The sheeting still resists the tips movement vertically as long as there is enough strength seperating the skins to prevent them from moving in relationship to each other. This is where the shear webs come in, they prevent the wing skins from collapsing. By incorparating diagonal ribs, we are allowing the strength of the rib to align more closely to the expected axis of rotation,(twist as well as fore and aft flex, tip moving front to back)
This principle is also why putting glass cloth or carbon fiber at the center of a structure will Not increase its stiffness, it will increase its strength with regards to flexability. In order to increase the stiffness of a structure you need for the material you are using to be the maximum distance from the axis of rotation, (the center of the structure in most cases) which increases the levarage, or moment arm that is acting on it. so in other words, if you want to use carbon fiber or glass to stiffen a structure, it needs to be as far towards the outside as allowed by the structure. If you want to increase the ability to flex and not fracture, then put it at the exact origin of movement, (or axis of rotation ) To hopefully make this more clear, say for example a bow, as in bow and arrow, you want the bow to be strong, but yet flex. the more of the glass reinforcement you put on the outside of the bow the more it will resist bending. the higher percentage that remains in the core of the bow the more flexible it will be. the arrow is a hollow shaft of composite because it needs to be very rigid, therefore any weight in the center of the shaft contributes little for rigidity(versus its added weight) so it is eliminated.