How wide are the tips? You can tiller from the sides to make them work more.
The ideal solution is to chase a new ring on the back, as this will reduce sapwood thickness all over. I do it very often with yew bows - leave it reasonably thick to start with, in order to see how the bow behaves, and once I'm happy I take the back down more.
If you're not comfortable doing that, don't worry too much about tillering the belly at the tips - you might end up with mostly sapwood there, but that's not the end of the world. Or you could knock down a few rings at the tips - say 8" from the tips down to the ends - and feather the violations back along the bow.
Yew will take pretty much anything you throw at it, so just pick the option that appeals to you most