We're in the process of remodeling a small master bath. We removed the old fiberglass stand-up shower unit and are going to put in a new tiled one. Floor area of the shower is roughly 3' by 4.5'

The plumber put in a new drain but it current comes up flush with the subfloor. His idea was that you then put in a membrane and then create your mortar slope and tile over it.

I had read that you want to create a pre-slope first - then put in the membrane and then continue with mortar and tile on top of that.

How vital is the pre-slope? My understand is if water gets through the 2" of tile and mortar and hits the membrane - if it is sloped it'll help channel water down to the drain area...