IMO it's an aim not a necessity.
I've several Yew ELB where I've had to drop down a good few rings (about 8!
) where the sap/heart boundary didn't follow a ring.
Mind the sap wood is plenty thick enough.
One of 'em is over 40 years old and has survived being overdrawn and refurbished with heat treatment to the belly... the back is still sound.
Here it is before the refurb', you can see the bug dip in the heart/sap... If I hadn't reduced the sapwood I'd have had no heart wood on the belly... it's always about compromise... bit like marriage (Yes Dear
)
Del