Sides are fine, if not preferred. There's much more adhesive area to absorb shear loads on the sides of the spars and webs than the edge of the web to the spars.

While I-beams represent an ideal cross section in bending (short of a box spar), I would argue a C-spar is best in our case. In our implementations, we have to rely on adhesive to provide the shear strength, and the more surface area for adhesive the better for strength. If I had an extruded aluminum spar, by all means give me an I-beam, but for a glued spar structure make it a C-beam.