i only use dishwasher soap to clean my car. heres the deal: the PO of my e30 (my dad) has owned the car since 1990. the only thing he ever used to clean it with was dishwashing soap. it has some of the best paint youll find on a 18 year old car.
even if you dont like using it all the time, ive heard its at least good to use it before a wax to help get everything off.