I have never left any sapwood on I have had heard of a few who have did it,,pure sapwood is weak and brittle,,if you have ever noticed when working old osage the sapwood is brittle ,,I think the sapwood that has made good bows,,is the last layer of sapwood before the hart wood,,there is usually one layer that is half sapwood and half wood, if you have ever noticed when you work down to a ring and it Is orange in places and white in places but is the same ring; I usually take off an extra layer to make sure all sapwood is removed,