Shower Pan Liner Pre Slope Installation.

I just wanted to go over the shower liner installation process since some contractors seem to think that a pre slope is not really necessary before installing the shower liner.
I just recently saw on You Tube what appeared to be a reliable and long term contractor stating that he doesn't deem pre sloping to be necessary in the shower liner installation process. Apart from the fact that he has convinced himself that it's not necessary, he went to construction sites and filmed new construction homes where the shower liners were already installed without a pre slope and used that to prove his point. Basically his reasoning is if those city inspectors are letting this pass then it's OK and it's not necessary.
First of all it is required but the problem is that the city inspectors in some areas aren't really checking for pre slopes which they should be.
We use the Oatey brand shower liner and if you look at their installation instructions it's saying you should pre slope the floor 1/4 inch per foot before installing the shower liner. What happens when you don't pre slope the shower liner is the water is not running to the drain as soon as it finds it's way to the liner. Instead what will happen is the water will run first to the lowest areas on the liner and then go to higher areas as more water comes in. The problem with that is first of all the water will always tend to be sitting there in those lower spots since it's not going to the drain. The other problem is as you're taking a shower now more water is going into the liner and going to higher levels which can go past the height that you installed your liner around your shower walls, the liner installation at the shower curb and the opening at the shower drain. So why take the chance of letting that water reach the first two than the opening of the shower drain? It's very simple, you should always do a pre slope to the sower drain before the liner installation. Basically some of these contractors don't like to do it because you have to wait until the concrete dries before installing the liner which means the next day and they don't want to have to wait so they can get done faster.  Below is a typical pre slope to the shower drain and then the shower liner installation.