Tank level
As Mog mentioned, I concur. The flushness of the tank stand is to prevent irregular stress along the tank's bottom perimeter; the bottom of the tank in smaller tanks particularly having the front/rear/sides resting thereupon.
The bottom glass can twist and bend to some degree (to accommodate irregularities in the supporting surface), but the seal formed between the silicone(typically) between the sides/front/rear panels can be compromised by distortion of the bottom pane, given that the bottom may distort slightly to accomodate the stand, whereas the sides/front/rear are essentially rigid, and are resting upon the bottom piece of glass. Distortions in the shape of the bottom panel are not readily accomodated by the front/rear/sides, and this can cause irregular stress on the seal between these panels and the bottom panel.
To clarify, any irregularities in the stand, may cause twisting force on the bottom panel which supports the side/front/rear panels of glass (of smaller tanks at least); the sides/front/rear don't give to loading so much, but only bow under water pressure, thus the potential for irregular stress particularly at the corner joints exists, at the bottom of the tank. No need to panic, it may simply lead to premature failure, rather than a blowout.
Numerous posters have mentioned the use of Styrofoam and such underneath the perimeter of the tank, to compensate for stands which are not flat, for this reason. This is not to level the tank, but to evenly distribute the stress underneath the tank. Level or not, the stand must be flush to the bottom of the tank, or the tank flush to the stand. Tilting of the tank imposes negligible stress on the tank itself; it should nonetheless, be resting on a flat surface. If not flat, adding Styrofoam to crush out minor defects is a good workaround. Tanks tilted, be not concerned, unless the stand which supports them is messed up.
PS, try slipping a playing card under the edges of your tank if you cannot see visual gaps between the stand and the bottom tank perimeter. If there are measurable gaps, you may elect to do something about them, but I leave that at your discretion. Styrofoam is cheap though...
DHM