Dish soap is for dishes. Car soap is for cars. Just because people used to do it doesn't mean that it's the right thing to do.
Quote:
Ok to wash car?
Question
Can I wash my car with dishwashing detergent?
Answer
Your car surface and the dirt that gets on it are a lot different from the food soils and dishes that dishwashing liquids clean effectively. We don't recommend them for cleaning your car.
web page -> click on Q&A up top, search for car.
Even Dawn says not to use Dawn on the car.
Good to use. 
