It doesn't matter how much water the owner would have to cross. No one wants to have a house designed and built only to find water pooling on the inside of the house. Most contractors will follow the plans. If the design is wrong they can always point the finger back at the architect and cite "poor design" or "poor choice of materials". A few contractors who really care about their work will at least attempt to bring a potential problem to the attention of the architect prior to doing the installation. They certainly don't want to rip out completed work and do it over again.