National building codes require the waterproofing layer to be sloped to the drain. The tile is not the waterproofing layer. If the tile was a perfect waterproof layer, why would it need a liner and weepholes? So, while the ignorant inspectors might pass it, those that actually know and understand the code would not. A properly built shower wouldn't leak even before the tile was installed - it would get gunked up with soap and crud, but it wouldn't leak. The tile makes cleanup possible and is a much better wear surface!
If you want to avoid the preslope, consider a surface membrane with a bonded drain such as the Kerdi system from www.schluter.com. Here, the waterproofing layer is on top of the sloped bed and then the tile is applied directly to the waterproofing layer. There are other similar systems, but Kerdi has been around longer, especially when you consider its origination in Europe. Noble company makes some nice tileable shower systems as do others.
Without a preslope in a conventional shower, it typically takes about 3-5 years to fully saturate and the pH to change so that that moisture will support growths - then, if used constantly, it will start to smell like a swamp. Being fully saturated, it can also wick up the walls and the floor will stay damp, aggravating mold and mildew growth. You don't get that with a properly built shower. One completed with something like Kerdi means there's very little beneath the tile that can get wet (only a thin layer of thinset), so the whole thing dries out much faster.
With surface membranes, some manufacturers provide various sized foam, tileable (after covering with the membrane) pans, but all of them can be used on top of a single presloped mudbed. This is more expensive in materials, but maybe not when you add the labor. Those foam parts only work well if your shower size and drain location are exactly in alignment with your needs (although they can be cut down, it should be done symmetrically, and that still may not suit your conditions).