If you're going to use the center AF point only...

Rule of thirds isn't a rule, this is a misconception. It's something which comes up in tutorials about composition and it's good because it makes you think. But for every example of a successful rule of thirds composition there will be many more which work in some completely different way. I think "cognitive dissonance" is at work here sometimes - people will see a shot which happens to have a strong visual element on a thirds boundary and declare that rule of thirds works, while conveniently ignoring other visual elements in the same photograph as well as all the other compositions which don't demonstrate rule of thirds at all.

It's always intrigued me that the layout of AF points tends to follow a diamond pattern. I don't think either composition rules or practical experience suggest that sort of spread, so I don't know why it's commonly adopted.