From BlenderWiki

Poles and flows

The Es cannot be eliminated but the Ns can be. Since we can't really eliminate poles in general what we can do is hide them and eventually they will be gone and that is the trick.

A good example of this is the NPoles for the forehead and the back (if you started with a Box that is). For the forehead you hide it inside the eye and for the back you hide it inside the Ear. You do it via a method called “UnPole” which I will go in-depth later. Now, just because you can remove NPoles doesn't mean you should do it! Some Npoles are meant to be there like the Nose and when you get into detailing the muscles for the human body you will get a lot of NPoles/EPoles since Poles control flows. A model with complicated flows will have a lot of Poles and vice-versa.

I have argued with a much more experienced modeler about the Npole for the forehead. He told me having the Npole there gives better control for the forehead. Now think about that for a moment.. we all know that Poles make our meshs bumpy. Better control with a price and that price is “Bumpiness” and we all know that the forehead is not 100% smooth! (Underneath is a skull) So yes, you can leave the Npole there for better control of the forehead. Whether you should leave that Npole there or not is up to you. To say “Should I remove it” is the same as saying “Can I use Ngon?” And the answer is always “If that is what you want, then that is what you must do. There are no rules.” The picture will get clearer once I talk about smooth/bump later.

When you extrude a poly you instantly created a circular flow (dark orange in image). In some cases it's good to have a circular flow with Es on the same lane (yellow), however in the case of a human head it's very bad!

(A): According to the screenshots of human head from professional artists in my collection, this Loop for the mouth is important. You cannot achieve this loop with both Es on the same lane so the trick is to move the upper E to the left lane (B). The next time you see both Es on the same lane you can be sure that it will form a Circular Loop.. If you remove (shifting it elsewhere) the upper E you will break the circular Loop like you are seeing it here.

Instead of shifting the upper E I shifted the bottom and this is the result. Pay attention to poles because they are your guide to better flows!

Here are two more images (below) to confirm that when you see two Es on the same lane you can be sure that it will form a Circular Loop.

Pay attention to the Es because by shifting one back and forth you can achieve “predictable” flows! There are no guesswork here.

You can use this dot/curve image as guide.