Erm...I just wouldnt weight the teeth, just no way. Parenting them to the highest joint in the head/neck is far more appropriate and will be lighter to use, this is how I do our set up in our studio. Teeth have no need for weighting as they do not deform like limbs, a simple rotation key with the correct blendshape ie mouth_open is enough.

things like the tongue can be weighted nicely with some nice spline IK though...

Cheers
Jay