John, if you are putting in a home made shower pan, and not aone piecefactory made pan with awhole in the middle for a drain, don’t forget that you must istall a vinyl type panning material and a specially made drain which compresses onto the vinyl material and the center screws up to the height of the mortar and tile. As for your original question it won’t hurt at all if the wallboard(prefferably greenboard orwonderboard,or concreteboard meets the mortar in the floor. Tile should be glued to the board all the way down to the base anyway. HOpe everything turns out OK!