Like the guys say, it's nice to follow a ring but it's not essential.
On very fine grained Yew it will be almost impossible, half of the trick is to get the light right to show up the subtle colour change vetween rings. Get about the right thickness of sapwood by roughing out with a draw knife and then spokeshave set for a V fine cut.
Then as you gradually work down the rest of the bow to nearer it's final dimensions there is less wood to worry about and you can tidy up the back by degrees with scraper etc depending on how pretty you want it to look
.
Violations running along the length of the limb or at a shallow angle are prefferable.
First few pics on this thread show the finish I get to, there are plenty of rings showing, but they are still hard to spot!
http://www.primitivearcher.com/smf/index.php/topic,31644.0.htmlDel