stacking rocks

I've always been careful to avoid having rocks up against the glass, simply because I don't want the glass scratched and it's a bear to clean well with rocks in the way. I think that a few rocks touching would be okay--the glass should be strong enough to withstand the pressure, but I would avoid having an entire stack leaning on the side. It probably wouldn't break the tank, but it might mean that the next errant tap will break it.