I don't use growth rings for anything other than a secondary or tertiary reference when tillering. it has been my experience when you can manage to center the crown of your staves back correctly and follow any side to side undulations on the grain correctly, (both of which you should always strive to do, imo) growth rings will feather out down the middle of the limbs by default.
from the other direction, if growth rings don't feather out pretty much down the center of a bows limbs, you didn't align the crown on the back properly or didn't follow the grain. both of these things are considerations that need to be addressed when laying out the bow, not when you get to the tillering phase. by then, it's too late. while it isn't always possible(for a variety of reasons) to nail the crown and follow the grain religiously, it can be done satisfactorily most of the time.