I have left practically every single one of mine rectangular shaped (core and belly piece)... same on standard BBO bows. There's simply no need to narrow them, which makes doing so a wasted step.
I keep wooden backing pieces such as hickory rectangular, but to facilitate proper thinning, I do shape bamboo backing pieces to oversized bow dimensions... the limbs, not the handle. I leave the entire handle area full width from flare to flare for layout wiggle room and because there's no need, wasted step... it will be cut to shape after the handle piece is glued on, so why do it twice?
I don't use the toothpick method, but rather, align the pieces with alignment marks on my jig. That way I'm sure nothing moved out of alignment as I bent the pieces down into the form. The more deflex, reflex or recurve induced, the more care should be taken.