For optimal performance and comfort, having a level toilet is recommended. However, floors, particularly in older buildings, sometimes have high and low spots that cause the toilet to rock or not be level. First, be sure the outlet horn of the bowl is not sitting on the flange. (If it is on the flange, you will probably need a new wax seal.) Second, to ensure the toilet is level, use a carpenter's level throughout the installation process. Third, use permanent shims between the bowl and the floor to level the toilet.