Hi coaster 500,
I've made several West Coast Style bows, so I can speak from experience.
I would absolutely Decrown the billet until the Sapwood only shows along the edges. The majority of the bow should be heartwood since the heartwood is lighter, and the Sinew is heavy, it's best to get rid of as much sapwood as possible.
I have chased the first growth ring of heartwood before, and had excellent results, however I think that this was unecessary and probably just extra work. Yew can be badly violated on the backing, and still produce sub 50lb self bows.
http://www.youtube.com/watch?v=N7E7zekzjOkThat is a 5 Minute Video of me making a Sinew Backed Yew Shorty. I hope that Helps.