Good question.
I think there's more than one answer to this.
I don't think there's any correct orientation of verts that would bend a curved edge correctly.
I would use two straight lines (with rounded ends) as the theoretical upper and lower lip, then bend them from there. The more verts, the smoother the bend.
That many bones isn't necessary for smooth bends but you could want them for controllability.
Still, the obvious triangulation (even with a hundred verts) would also annoy me to no end at this size, so I would actually give up on using PathConstraint and just do swaps and little deformations between predetermined mouth shapes, just for the sake of getting the edges to look perfectly curved.
It still makes me sad that cubic spline shapes still aren't a primitive in graphics api/game engine terms. Everything has to be hacked into meshes.
Maybe other people have better ideas.
