How much of a gap are we talking about in the center?
I think All-Glass and Oceanic stands only support the end panels of the tank. I had a 5' Oceanic stand a while back and there was a considerable 'dip' in the center (.030" or so). If I remember correctly, I did some research and this was normal. In theory, that gap could be .03" or 3' and it wouldn't make a difference, but I'd feel better if it was a small gap.
I would imagine you're much better off having most, if not all, of the weight supported by the ends as it would be much less prone to flexing the tank.