It doesn't really matter. I've made several with oval section limbs where the sapwood wraps rounds a tad, it just vies a nice coffee and cream look on the belly (or side).
I'd say keeping a bit of width may be more important to the stability and tiller of the bow.
You don't need to trap the back as it won't overpower the belly... the sapwood and heartwood have been married for so long they know how to put up with each other
Del