I think the sap wood is just not a predictable as the heartwood,, that is why bowers take it off,, if you dont have enough wood then you dont have anything to loose by trying,, the sapwood on smaller saplings seems to be more stable.... also in my experience the sapwood checks a little easier ,, so taking it off eliminates some of the that risk,,, an good osage bow is hard to make ,, so many eliminate it to increase the odds of success,,, I have a very good bow with sap wood,,, and have seen others that were great,, if you try to make one with the bark on,, you will see very quickly why that is not a common practice,, leaving the sap wood on,,,, is not a problem if you area going to sinew back,,,,