I will be remodeling my master bath soon and I will be putting down 1/4" Hardibacker on top of the subfloor to lay the tile on. Since the tub will be replaced, should I lay the Hardibacker under the tub as well? This was the mortar bed wont be directly on the plywood subfloor? Any issues with doing this as opposed to stopping the Hardibacker where the tub begins.