With yew, when you are looking at the bow from the side and the sapwood takes a little dive or makes a bump, then you should follow it with the heartwood. If the sapwood takes a bump up, then follow with the heartwood. If you don't, and just make the heartwood flat, the bow will be very hard to achieve even tiller. Try to maintain an even ratio of sap/heartwood for the full length of the stave.