I'm currently retopologizing a sculpt of a snake with an open mouth, before adding the details in a multires modifier. The edge of the mouth is in some places very thin, and I'm wondering what the best practice is for this. Should I have an extra face on top of that edge? (top of my example image) Or just an edge? (bottom of my image)? 
It's better to have square instead of elongated faces. I remember JL or Kent advising on having faces as squared as possible, when they have that elongated shape they do cause a bit of trouble from time to time. But it's not a rule set in stone, you can save up geometry and not have loops going all over if they are not needed.
