When making a portrait, set the focus on the eyes, then recompose.I always use a single focus point when photographing still subjects. Most of the time I keep that single focus point in the center, and the recompose how I want the final image to look. Of course for shooting still subjects I use AF-S (one shot) focus mode. So when making portraits using the settings described previously, I set the focus on the subject's eyes, then recompose because the most important thing with a portrait is getting the eyes in focus.
Very rarely do I center subjects, and in a portrait the eyes of the subject are not very likely to end up perfectly centered as well. In the above shot I first put my center focus point on the subject's eyes, then recomposed to put her eyes in the upper third of the frame (also her right eye ends exactly at the right horizontal third).
So when making portraits, set the focus on the eyes, then recompose how you like.