By all rights the tank should be dead level but in all practicality that is not practical in most cases. I have my large tank in my basement and by design concrete floors are not level so I had to shim the tank stand along one side. This brought the tank within my allowable differences less than 1/4 " over 5 foot tank is OK as long as the stand is not twisting and bending. You should have Styrofoam under the tank any ways so it will take up any twist of the stand instead of your tank or seal breaking.

Bill