Prejudices can be intertwined with instructions because the writer is giving you ALL of his opinions on how to manufacture the weapon anyway. If he had tried to make an ELB from ash, and inmmediately strung it up like he would have with yew and Osage, the bow would take too much set. We know that now. Likewise, if we rush bows trhough tillering, or don't design them rpoperly, the sapwood would provide a spot for the bow to break. Personally, I think leaving the sapwood intact is fine- it insures one ring and simply reduces the mass of the piece. Since many of us use less dense woods anyway, why should it make that big a difference? I think if one goes slowly enough, and watches the bow as it forms, then the chances of its survival, and power, greatly increase.
Anyway, there's only one way to know for sure, and that's to try. If it fails, oh well.