Yew isn't so fussy about violating rings on the back. Obviously do it as little as possible and preferably in areas where it isn't so important.
E.G On my big old fave Yew ELB I had to remove a fair bit of sap wood at one point, the rest of the back is just the underbark surface.
The Yew primitive I'm working on now is being decroned in parts for the same reason.
Have a look at my Bowyers Diary there's a pic.
I'll post a pic of the ELB tip if, just hang on a minute!.
Not the best pic... conventional wisdom would say that back was horrific and would fail. I've had that bow for about 30 years and it's been back to 90# at 31.5" on occaision.
That is actually mid limb!!! There is a big dip in the sap wood there and not much heart, so I relieved the sapwood, see pic below.
Given the choice I felt going through 8 sap rings whas better than having a thin point in the heart wood.
There is a fair bit of sapwood taken off at one tip too, for the same reasons.
Maybe I'm just lucky, but with a stave you just have to go with your gut...
Hope this sets your mind at ease a tad
Del
BTW, that's the bow on my home page if you want to see it at at full draw (google Delsbows)