All these shortcuts I hear scare me. No matter how much waterproofing is done on the surface of the poured pan, some water will weep down either near the drain or through the concrete itself. Concrete is porous, grout is porous, and wood loves to soak that water up and turn it into mold and mush. I know there are other methods out there, but I prefer the method of using a pvc shower pan liner directly on the subfloor with a screw up drain. You can notice that the screw up drain has weep holes so water that gets through the shower floor and rests on top of the liner can drain.
The idea of pre-slope, liner, then mud bed works, but isn't entirely necessary. Depending on building codes, the shower pan liner must go up a minimum of 2" over the top finished height of the shower curb. Tucking the corners is tricky, but there should be absolutely no holes, cuts, or screws below 2" from the top of the curb. The liner is installed before the backer board.(remember that it's back there when screwing on the backer board and avoid putting screws in the bottom 6" of the board)
Also, I always left at least a 3/4 " gap between the backer board and the floor pan to prevent moisture from the pan from wicking up the wall. The tile would go all the way to the floor, but the bottom of the backer board would never be fully submerged in water.
Hope some of this helps.