Date: 4/8/2008 11:33 pm EDT
The short answer is it depends upon the flatness of your plywood floor, and the thickness. The subfloor should be at least 3/4" plywood. As a rule of thumb, if the tile "rocks" the floor is not flat enough. However, it will generally be easier to apply a self-leveling compound to low areas and knock-down any high areas than install a backerboard.