Instead of a wax ring for toilet over tile installation, try either a Fernco 'Wax Free Toilet Seat' which you can view at: http://www.fernco.com
or a FludidMaster Wax Bowl Gasket: http://www.fluidmaster.com/connect_products_7500.html
These are especially helpful in mounting a toilet over tiles where the level of the surface may be in question.
Wax free toilet gaskets allow for some movement of the the toilet without breaking the seal as is the case with a wax ring.