Well, selecting a focus point to cover your subject is the "proper" technique. Focus and Recompose will always throw your subject out of focus, unless you have enough depth of field to make up for the shift in the focus plane you are introducing.


The problem is that focus points tend to stay away from the perimeter of the frame (especially on full-frame), this is due to the fundamental design of our SLR's mirror box. The secondary mirror has to get enough of a down-angle in order to reflect light down into the focus sensors, that means it must be smaller (and attach lower on the primary mirror) to avoid hitting the sensor/film-plane. Now, this really only affects top-bottom, so I guess it would be possible to go left-right to the edges, but they don't AFAIK.


So what to do? I would rate the various methods I am aware of from best to worst like this:


1. If using a tripod is an option, (i.e. for a landscape or static scene) I would recommend using live-view, and moving the little zoom box to hone-in on your subject, then manual focus using the zoomed-in view.


2. If you can reach the subject with a focus point, select that focus point, then focus.


3. If you must use focus & recompose (for speed), try to increase depth of field, or just be aware that you are moving the focus plane, and try to compensate somehow. (move the camera or slightly or move the focus ring, although this is probably as risky as not trying to compensate at all!)


Finally, remember you need a lens that is sharp in the corners if you will be putting a subject there. If your lens is only sharp in the center, you are better off always centering your subject, then cropping to suit composition, provided you have the resolution. Also remember the outside focus points vary in quality depending on the body. Your best focus points are usually in the center.


IMHO