Just in case it is important in decision making, I should point out that I wash the cars about twice a month from April through September (good weather in Seattle) and maybe once a month if that from October to March (the rainy season in Seattle). I do not stand out in the rain to wash the cars solely for the sake of washing them. Someone in sunnier climes who washes the car once a week may see different durability results than mine.
PS: I also wash the cars by hand at home using some sort of TW wash and wax that costs $ 8/gallon at Wal-Mart. I do not run the cars through a commercial or tunnel car wash.