I remember about a year ago on a website (think it was chichlid-forum.com) where there was a discussion about the role of the center brace/glass bowing and someone that had a tank they were going to repair the center brace on actually took pics with a tape measure on the tank both empty and full of water. The tank actually did show a solid 1/2" bow when it was full with no center brace.
It doesn't seem like glass would actually bow, but it is true I've seen it happen.