I agree that an imperfectly level floor is not a huge concern...up to a point. A nice flat floor that has a very minute slope in one direction only is not likely to be an issue. However, if the floor is...like my basement floor...a bit "wavy", with complex slopes at various angles to one another, then you can have a problem. If you put the tank/stand on the floor and it is leaning a bit towards one end, but is in complete contact at all four corners, you should be okay; but if the tank/stand sits in such a way that one corner is always off the floor, then the stresses this sets up can be disastrous. This situation is typical in basements that have floor drains, as the concrete is purposefully sloped towards the drain from all directions.
Personally, I would build the stand by attaching a perfectly level piece of wood to the wall, running the length of the stand, and then build the stand using that as a reference point. Attach the two end pieces and the front piece, temporarily supporting them so they are also perfectly level. Then carefully measure and cut each leg individually, making each the correct length to support its corner evenly and also rest hard on the floor. Having each leg a different length, to suit the discrepancies in your floor, requires building the stand in place, and requires a proper 4- or 6-foot carpenter's for accuracy. Additional braces, reinforcements and other pieces can then be easily added. Done this way, it doesn't matter how goofy the floor may be; the tank support surface will be absolutely flat and level, which is IMHO the ideal solution.
I've made a number of racks and stands this way to match the wonky floors in several basements. It's much easier done than described. The only negatives to this method are the fact that the construction must be done on location, and once the stand is completed, there is no moving it; it fits there and nowhere else.
