For that light a bow I wouldn't worry about leaving sapwood on as long as it's a good clean layer with no big knots or violations of grain (as you mentioned that it is fine). If it was a much heavier draw weight, like a Cherokee warbow (which is basically just a heavier built and longer version) or a much heavier, say 70#+ flatbow, I would chase a heartwood ring instead personally If it's a self bow, to be a little on the safer side.
If you really wanted to keep the heartwood contrast look with your bow design, you could work the sapwood down a little to thin it and keep more of the heartwood on, but that's probably more work than you would need to do.
Also you could try to make more of a longbow style instead of a flatbow style (for a narrower and deeper cross section), but again, your preference. I'm sure the design you have in mind will make a fine bow even if it's mostly sapwood.
