A leveling system is absolutely essential in a thin tile panel installation.  We recommend selecting a Large Format type thinset, back buttering of the panels with the trowel grains running north and south in the direction of the smallest dimension of the panel.  The floor/wall thinset grains should run in the same direction so as not to entrap air pockets.  Leveling Spacers should be placed no more than 24” apart, and approximately 2” from each corner.  Please do not put Leveling Spacers in the corners on a thin tile installation.  We have not had any reported problems, but it just make sense that thin tiles would break if you did not get the perfect thinset coverage below the surface.  It is not worth the potential risk.  Next, place the tile according to your preferences and work to eliminate any air pockets using your preferred method.  You will love the results.