I think VB is talking about BoKu and me... BoKu has airplanes in production. Listen to him. I am more nearly academic. If I live long enough I will fly the bird out in my shop...
When you bevel the foam, the shear web can become a hinge as was pointed out above. In addition to that the shear web right at the spar cap is the place where loads in the web are biggest. Adding the "hinge" deformation to the rest of the hard life the web material has right there sounds like really bad juju.
Some folks spend some effort interleaving some plies of shear web through the cap lamination... A man of no less import than Jim Marske trains folks to do this.
The other thing is that you will have hardpoints in the web someplace, and you will substitute something like phenolic plate for the web. Nice beefy cores at those spots make the transitions easier. Web cloth folding up and over bumps is also bad.
As to VB comment about making a rectangular beam, the best reason to do that is when a broad (chordwise) spar is being included to carry big torsion. These sorts of beams end up kind of overbuilt for other tasks, and that is extra weight, but they usually make that trade to avoid something else that they think would cost even more weight or other issues.
The nice channel spar is my preference for several reasons. The big one is that they are easier to make well - straight and well bonded together - and that is important. A poorly built and/or aligned spar can become a disaster many different ways. The other reasons for a channel are almost esoterica: Your thickest web lamination is inside the caps, while your thinner one is outside. In a full rectangular spar, all of the web plies go outside, pushing the caps inboard more than on a channel. Many only put two plies outside. This allows the caps to be further from the neutral axis, and so may allow lower weight. If you are of the persuasion that some web plies should be interleaved in the cap strips, it is easily done with channels too.
How do we carry all that torsion? You have to hang the main landing gear (yeah tricycle), flaps, ailerons and the like on a drag spar anyway. And in my case, I also have to close the fuel bays and have faired slots for the trailing edge devices. Might as well run the drag spar to the fuselage and carry torsion between the two spars...
Billski