If I am shaping a retrieve and my dog moves her mouth one inch from the dumbbell to half an inch, then I am closer to my goal.  I will mark and reward. But consider this. Rather than watching the dog's mouth in relation to the dumbbell, someone else might be watching the side of their dog's mouth or neck, looking for the muscles to tense as she begins the process of moving her head forward.  This makes sense because in order to move your head forward you have to tense your neck muscles. If you videotaped the dog within the training session, the two handlers could end up marking and rewarding at exactly the same moment in time but for different reasons.  They focused on different things. Which brings up an interesting question...what was the dog focusing on? There's no way to know!  Maybe the dog shifted her weight from the rear end to the front as she reached and that is what she noticed.  So now we have three possibilities.  Indeed, there were probably several more behaviors happening that we were not paying attention to. So what should we do, look at the dog's muzzle in relation to distance from the dumbbell, look at the movement of the dog's neck or mouth, or look at the dog's distribution of weight? It doesn't matter.   Pick what is easiest or most entertaining for you. When I consider the many ways to teach a dog scent articles, position changes, weave poles,  precision heeling, jumping, etc... at the end of the day, there really is no right or wrong, no "greater good", as long as the method is respectful to the dog and progresses the handler in the direction that they wish to go. Maybe the only greater good is the flexibility and willingness to change direction when you see that something is not working for the dog, the handler, or the trainer.