You don't have to follow one single growth ring on the back. Just make sure you don't violate toomany growth rings in a small section. What I am trying to say is that if you are going to violate growth rings, spread it out a bit.
Some people like to thin out the sapwood, so they get bigger heartwood to sapwood ratio.
Yew is pretty forgiving.
I made a 120lb at 30inches yew warbow last spring. I managed to saw a part of the mid bottomlimb to be half an inch thinner, making a dip in the taper of the limb. I just compensated it with more thickness in the belly and the bow is still shooting fine and the tiller is nice.
![](https://i.imgur.com/ddAhNSm.jpg)
![](https://i.imgur.com/LrNwnXL.jpg)
Cheers,