I usually supplement my standard shots of a person with a few extreme closeups of their face. Getting in tight and snapping a few frames of their smiling face usually produces a keeper or two. This is especially true if the subject has face paint or some other embellishments that are part of their costume such as glasses, jewelry, or homemade constructions.
Take shots of faces when there is interesting detail.
For example, I recall the time I was photographing a Na’vi from the movie Avatar. Imagine a completely blue face with silver speckles, false teeth, and long black hair in tight braids. Just her face had so much intricate detail it was worth capturing alone, so at times like that you might want to supplement your standard shots by getting in close. If you have time, examine their face a bit to look for the most flattering angle.
It is true that people usually have a viewing angle where they look their best, unless their face is perfectly symmetrical and none of their features are out of proportion. Once you have a good angle, take your shot, keeping in mind that larger apertures are generally better suited for images of the face because most lenses are slightly less sharp at maximum aperture. Sometimes you don’t want to show too much detail, that is unless it’s someone like the Na’vi cosplayer.