Sterling Esemble Tub

Its one of those plastic tubs that is not supposed to be set in mortar. (i am tiling the floor in this bathroom) My question is, Should I set it directly on the plywood subfloor or go ahead and lay the cement board down and set the tub on top of that?

If you have not already installed this tub, I would simply place it on the subfloor and then build everything else around it. I can't imagine what it would "buy" you to put cement board (material that provides no structural support) underneath - unless you are simply using it as a spacer.