If you use 3/8 or thicker styrofoam it should level itself out when the weight of the water hits it. I would definitely say that you should attept to level out the stand. We assume that the stands are built level but that's not always the case. If it's metal stand it will never be the case. IMO use the foam and get the leveling close. As stated earlier technology should make it so that you donor need to be perfect but at least try to get close for the sake of you tank, floor, and fish.