I worried a lot about this when I got my 5D.
After months of shooting with it (almost a year now), the point spread really doesn't bother me at all. I guess it's just an "adaptation" thing. The spread was better on my XSi, but I still get the shots I want, and when I'm stuck using the center point only, when re-compose isn't a good option, I'll crop the image slightly for improved composition.
Plus, the rule-of-thirds is a bit overblown, IMO. I was turned on to the diagonal method last year, and I think it's improved my compositions.
http://www.diagonalmethod.info/![]()
In fact, Lightroom has a crop overlay for the diagonal method.




