first, you don't NEED a trapezoidal cross section to keep from lifting splinters on the corners. simply chamfering the corners will do that, and I think its a good idea to always do it before you start doing any serious bending. I round off all the corners even after trapping the limbs.
I believe firmly that a trapezoidal cross section is beneficial to the design of most any wooden flatbow if done correctly, especially those with flat bellies and a backing of some sort. every selfbow and bamboo backed osage bow I've made in the last 10 years has a trapezoidal cross section, although it is much more pronounced on my bbo's with their "as flat as I can make them" bellies.
for lack of a more clever way to make the trapeziodal shape even on both sides of both limbs I use my bandsaw. I tilt the table to the desired angle and trim the edges with the belly flat on the table, clean it up with an 8" long sanding block with a piece of 36 grit paper from a spent sanding drum wrap glued on it. this works better for me than a rasp for "squaring up" the edges of limbs.