I rather doubt that using hot water is going to do any real harm to any wax applied to the vehicle. Even a white car will reach a surface temp of 120F or more on a hot, sunny day and dark colored cars can easily reach surface temps of 150F or more. Surely nobody is hand washing a car with water THAT hot?! I also don't think there's much to gain from washing with hot water either.

I'll check with our chemists at work this week and see what their take on this is.