Most of the load IS in the 45 degree direction when it comes to the actual primary function of webbing. However there's a second load direction that doesn't seem to receive the same attention. This is the tendency for the upper spar to want to buckle inwards when put under a lot of compression. For this aspect vertical grain is by far a better option.

Even better is where the webbing is located between the two spar caps and has a good fit as opposed to being glued to the front or back edges of the spar caps. And the thinner the wing the more important this aspect becomes. On a big aerobatic ship with a thick section you can easily get away with glueing to the faces. But on a thin sailplane wing the shallow spar cap spacing multiplies the effect of the same amount of G load and glue or balsa will easily fail if you're relying on just the joint to hold. Hence why sailplanes all go with vertical webbing located between the spars.