one reason making lip corners deform a more local area than a wider area is so can use rotations of the joint without affecting too much extra area. The side affect as can see is when the entire lip is translated down parts of the cheeks dont follow. This may be a reason to use corrective shapes for the lip up and down.
an overall weight painting tip is that the smooth brush can come in really handy when the selected area that needs smoothing only is affected by 2 joints. This may be a reason to try to keep to few influences per vertex.