I have heard two version on the effect of road salt on the car in winter. Version one says that it will corrode no matter how cold, the other version says that salt loses it's corrosive properties if it's cold enough (below freezing) outside.
To tell you the truth, I apply as much protection as I can during autumn because when winter comes around, I will definately not wash the car more than once a month or so. (I don't like car washes, even if they're "touchless")
Canadian folks will agree that it is way too cold and way too dirty outside. I don't have a garage (meaning I can't wash it myself) My car is my daily driver, so as soon as I drive off, it will get salty again in minutes, not to mention the headaches of frozen doors after a wash. The only part I clean regularily is the interior.