As others have said, following a ring is nice and a worthy aim, but not essential with Yew.
Do try to keep violations to a minimum and where you do have then , try to have them running along the limb or at an angle or like flames (like you see on the belly of a bow).
If you leave the back a little thicker than planned at first and then clean it up slow and steady one the bow is floor tillered, theres less work in following a ring.
My last bow here gives an idea of what I mean.
http://www.primitivearcher.com/smf/index.php/topic,28324.msg376269.html#msg376269And here's a pic of the back of my big old Yew ELB (75# @28"... 90# @31.5")
I followed a ring over most of the limb, but if I hadn't gone down about 7 rings I'd have had all sap and no heart where there was a sudden dip in the heartwood. (See below) Ithink the two pics aren't the same way round, but it shows why I reduced the sapwood at that point.
It shows that you need to make compromises from the ideal.
BTW the sapwood is thinner along the other edge as it isn't even across the bowalong the whole length.
Yew is beautiful to work but can start to tear as it follows the grain or near knots, if this happens go to a spokeshave or rasp rather than the drawknife.
Del