I have used mulberry sapwood with good results. I have made bows of almost all sapwood, about half and half and with just a ring or 2 left for a backing. If you know the sapwood is sound and was cared for properly, it will be fine on your bow...especially with a bow that is 72" long.
If you do leave some sapwood on, you bow will be a bit thicker than if it is just heartwood. Pat