Finger sucking isn't a guide for hunger. It's a non nutritive suckle to exercise the cheek muscles. It's a stage of fixation every baby goes through. It actually sooths n gratifies the baby. Please don't interfere the process else the baby continues to do so for a longtime. Usually they grow out of it in few months. If forced to stop it,  chances of cultivating habits like biting, smoking, bad mouthing etc increases in latter life. It's definitely not gonna make baby have buck teeth or pouty lips.