Quite often it is best to focus on your intended subject (and if its a person, on their eyes), then hold the focus and recompose the image to make the composition stronger - like having your subject to one side or the other rather than dead center.
Most modern dSLR's have multiple focus points available to get good focus on subjects in the middle as well as to the edges of photos. For most accurate focus it's often best to manually choose a focus point rather than let the camera choose one - an example would be focusing on the subject's eyes.
If you're working with a shallow Depth of Field (see Depth Of Field to learn more about this subject) and use the center point of a multiple point focusing system (common in most dSLRs) then the focus might actually be behind the subject because the 'focal plane' will swing on an arc. Its best to choose a focus point in your camera closest to where you'll want the subject positioned.
Please check out your camera manual to find out how to do that with your specific camera.
Here is an example - if you're shooting a full-length portrait, the best camera height is about waist level and the best focus is on the subject's eyes. (Yes, we've cut off the subject's legs in this illustration, but in a photo you'd show the legs as well.)
However as you swing the camera down to recompose it the focal plane swings in an arc and winds up about the subject's ears rather than the subject's eyes.