A couple reasons why its helpful to sculpt the gums and teeth before facial rigging are:
- For lip roll shapes. These facial shapes need the lips to be able to roll okay and expose teeth properly. With the teeth in place its easier to sculpt the inner lips so the lip rolls will look okay.
- prevent teeth collisions with lip narrow shapes. By modeling teeth first its easier to prevent later blendshape sculpts penetrating with teeth with narrow lip shapes.
- Aids in sculpting smiles etc. This is because for a smile the amount of teeth exposed hints at how much emphasis is on the smile shape.