Waterproofing is mandatory over here, but since customers generally want to get away as cheap as possible with materials, I charge labour in relation to application time and drying time.
New regs say waterproof membrane or waterproof substrate with sealed joints for domestic bathrooms, but until the 1st of July, you can get away with using the older, cheaper, roll on systems. [Sidenote: over here, the whole bathroom is tanked, not just around the shower and bath tub]
I discourage that by charging more labour for those, as most of them take almost a day to dry (especially the cheap ones), and I still need to get paid for the entire day you know.
The membrane systems are faster to apply, and dry alot faster.
Thus, I can usually begin to tile the same day I apply them, saving me time and allowing me to do more work in shorter time. There is little or no cost increase for the customer, even though the materials are much more expensive (about 30%). The end product is also better and more durable, which is good for business.
I use the same approach when leveling. I don't use cheap crap which dries 1-2mm per day; I use high quality gear which is tileable the same day, or the day after if it's in a bathroom. This approach allows me to focus on one job at a time, and that saves me stress, and gains me more money, as I have more chargeable time than I would otherwise.