I just read an old PA magazine(vol. 9, iss. 3, Fall 2001) with an article by Tim Baker; Bow Woods, Part II. Tim suggest that sap wood isn't bad and is usually stronger in tension than heartwood. I have found with osage, mulberry and other woods, a little sapwood, if properly handled after harvest, works well on a bow. He does suggest that sassafras along with ERC and cherry are tension weak woods but if properly designed and tillered will make a bow that will out shoot heavier woods bows(due to physical weight). He also suggests a slightly rounded belly for these lighter woods.
One of the first bows I ever made was a sass bow. I have a small diameter(6") tree picked out to cut for bows but haven't cut it yet. Looking forward to seeing your bow. Pat