I've never worked with yew myself but, from what I've read, it's supposed to be more than adequate without the sapwood. I read somewhere that the bow would have to be way over stressed for the difference in strength to come into play. If it were me, I'd probably work it down to the first good ring, be it heartwood or sapwood, and go from there. I could be wrong. Like I said, I'm just repeating what I've read. Maybe we'll hear from someone who knows what there talking about.
Josh