I, too, wonder that the optimal cross section might be.
Anyway, to add some data for the OP; I've had an intentionally under-built, unfinished bow of around 100 pounds that measured 30x30 mm in the center with a perfectly rectangular cross section and only slightly rounded corners. It took no set but failed in tension around 29, 30 inches, I presume due to the corners on the back not being rounded enough and a botched heat-treatment that weakened the back.
Regarding the belly cross sections I think that the flatter the belly the more durable the bow and thus you should make it as flat as possible without having a terribly uncomfortable bow and (this is important, I think) creating abrupt transitions, "steps" in thickness taper due to the earlywood being soft/spongy. I've had this problem with my two rectangular ash bows.
On top of that they were ugly, the rectangles.
I think that the cross section should allow for easier thickness tapering and wood removal as well as a more comfortable grip. I plan to make my next bow fairly rounded - a bit less than your stereotypical MR bow.