I'm a bit late in this thread but to illustrate the problem of having a tank that is not level, picture this:
In a perfectly level tank, the weight of the water (downward pressure) is supported by the bottom glass and whatever is underneath. All sides are perfectly vertical so they, along with the silicone seals, only support the sidewards pressure which is much less than the downwards one I would imagine.
Tilt the tank forwards slightly, and the situation changes - now the front panel not only has to support the sidewards pressure, but the bottom silicone seal now also has to support the downwards pressure of the water that rests on the slightly tilted front panel.
Now tilt the tank to the left - you now have the same situation on the front and left panel. So the bottom silicone seal at the point where those two meet is under a LOT more stress. This will be the point of failure eventually.
How long it will take depends on how well the tank is built and the degree of tilt - but that point will eventually fail and you either end up with a slow leak through the corner or worse, a corner that fails catastrophically with potentially shattering glass.
Getting a tank perfectly level is almost impossible - but try and get it as best as you can, to minimise the forces that those side panels and their silicone seals are under. The higher the tank, the more important it is because higher = more weight of water.