Where the sapwood ring count is low enough to be followed leave the fatter white wood late growth untouched.
Incidentally, if you want to make a flatbow with sapwood on the back, it is useful to have a stave with a thin sapwood layer which can be left pretty much untouched.
On the other hand for a longbow a thicker sapwood layer can be reduced pretty much by eye, considering the character of the stave itself and the sapwood/heartwood interface, bearing in mid that this interface might not show the same appearance on both sides of the stave.
Rod.