Before you go ahead and extrude all the way around the mouth, stop and take a look at what it is you're doing. If both images lined up well and were drawn by someone who did this kind of thing every day (ie., not me) then you'd probably be able to just trudge along with no worry about odd shapes happening as you model. However, I'm no expert at turn-around images, so if you were to just continue on following the lips in both drawings, you'd end up with a very point mouth. To get around this, I'd suggest concentrating the bulk of your modeling the mouth in the front view. This will force you to spread the mouth out a bit until you reach the corners, which would be desirable.