I noticed almost everybody models the mouth closed, lips sealed. The problem I have with this, is that this way makes it more difficult to open the mouth for animation, such as using soft selection to move the upper lip away from the lower one without having the lower lip verts get dragged up too (and vice versa). So that…
You can also weight the head to several helper bones like a jaw bone that way you're just moving a bone around every time you want to open the mouth. Then clone the mesh to make a new target shape.