You have to learn DoF.
www.dofmaster.com/dofjs.html
I'm focusing on the part of the body which I want to be in focus.
With portrait orientation it is easy on Rebel. Very top AF point or two under it are usually closer to the face, where you want usually focus to be.
In landcape your seater face is more often not even close to AF point and this is why it is often more difficult to focus.
Usually I focus with closest AF pont and recompose for portraits if DoF permits, sometimes I have to MF because DoF is very shallow.

all my photos look like this..

Canon Speedlite 430ex ii 
