If you subscribe to the idea that you need a pre-slope below your water barrier, than you should install a top layer that consists of a cement mortar mix. The top layer serves two purposes, first, it will protect your water barrier from damage that can be inflicted by foot traffic or trade error, second, it will provide a surface for which your tile will adhere to. The top layer will follow the same pitch that was set by the pre-slope, encouraging moisture to reach the shower drain.
The top layer is not a redundant installation, but there are some that would say otherwise. It is important to remember that you should have already established a strategy for your built up shower pan, therefore, you either believe that a pre-slope is an important feature or you already determined that it was not necessary. In either case, the top layer should follow the same rules, in the event that the water barrier does not have a slope, the top layer pitch should be 1/4 inches in height per foot leading to the shower drain.
A built up shower pan is not only installed to encourage moisture to reach the shower drain, but also to allow for moisture that remains in between each layer to evaporate. Moisture that remains embedded within the layers of the shower pan will provide an opportunity for mold to grow. If a top layer sits over a water barrier and a second water barrier is applied or installed over it, there is a chance that moisture would not properly evaporate. The common term is a “moisture sandwich”, indicating a layer that is has a barrier above and below it. There is an argument that moisture should never reach below the secondary water barrier that sits above the top layer, but the reality is, during normal usage, it can not be guaranteed that moisture will not reach below through cracks that are formed over time. The best method is to choose your water barrier to be installed below or above the top layer, but not both, in that case you would need to rethink your waterproofing strategy for built up shower pan.