if i built my own cabinet, i'd probably want to level the stand as well as the tank. that may be reflecting on my cabinet making skills, but you don't want the box subject to unnecessary stress either. i'd put plywood under the stand, shim that level, put the stand on it, and then shim between the plywood top and the stand to level the tank.
i'd also recommend putting some foam between the tank and plywood, to perfect your leveling job. i used the closed cell interlocking foam panels they sell as "anti-fatigue" flooring at BJ's. it's cheap and effective, unobtrusively black, and your excess is useful for silencing pumps etc.