From Canon Digital Learning Center
"Everyone has a method of getting critical focus for a shot. There are no right or wrong ways to do it. If you can get the focus you are looking for, then you are doing it the right way for YOU.
Canon’s engineers have included a plethora of tools in recent EOS cameras to aid the photographer in getting that perfect focus. They have included great features such as more autofocus points, smaller Spot type focus points, three options for expanding the size of an AF point, and even a joystick to control them. While each AF Area setting has its place in different shooting situations, what if you want to use two interchangeably from shot to shot without slowing down?"
See the entire article on the Canon Digital Learning Center