You don't HAVE to leave ANY of the limb stiff and non-bending if you don't want to or need to. I usually push the limits of design and wood ability and want every inch of the limbs to do their share of the work. If the ends of the limbs work no more than any other part of the limb, there's no way it's going to break because of the work it's doing.
Any limb wood that isn't working needs carried along for the ride, and the more that doesn't bend, the more stress it applies to the area of limb that IS working.
I generally tiller the limbs to work right up to the string groove.