Emmet,
I dont believe there is a definitive answer here is why. In every case where certain tribes and or bands had scalloped bows, they also had plain sided (rectangular section). In nearly every instance the plain outnumber the scalloped by quite a margin.
As far as I know, the shape and decoration of a bow was strictly up to the caprice of the carrier (owner). Having said that the bows that are scalloped were done in certain patterns affiliated with the particular tribe or band,i.e. An Odawa bow looked different from a Ojibwa which was different from a Pottawatomie......even though they were related brothers.....then all of theirs were different from the Oneida and different from the Mohawk, even those were brothers....so my inclination is that the scalloped bows were decorative, and also signposts while at the same time being fully functional bows.
I have used scallops to help tiller out a few bows but do not believe that is the main purpose, rather a matter of "look at me, I'm special" for whatever reason.
rich