I've seen a 180g tank on a metal frame before - with a bad bow in the front beam of the stand which was probably at least a 1/4 inch gap. It wasn't leaking, but you can bet that the silicone holding the bottom glass to the sides was holding some stress that it wasn't intended to....
That kind of stress can obviously hold for a while, but it'll sure shorten the lifespan of the tank!
(It was here in Calgary at Riverfront....I wonder if it's still there...)
Same problem as with a twisted aquarium stand frame.
Mitch