It's more important that the tank be on a flat surface than a level surface. Not level can lead to issues like where a hang on filter should go or seeing a water line at the top of the tank but on a non flat surface, that could cause the tank to crack. One way to compensate for non flat is to put polystyrene or styrofoam under the tank so that any pressure points would be able to move on the styro. For unlevel on carpet, you can put the stand on a piece of plywood and then shim the legs so that the stand is level then use the styro and then the tank. That should handle all the issues except the floor itself. If the floor is unstable, I would seriously consider putting the tank elsewhere or supporting the floor from underneath.
Hope this helps