I have never seen anyone use sand. I always use mortar. The morter is sloped to the drain flange, and the membrane is attached to the walls and the flange laid on top of the mortar. Not to say that you couldn't use sand, if someone can prove to me that the sand will support the layer of concrete on top of the membrane with out cracking the tile. I would have to see some proof. And know the reasoning that this way is better than the old way.