I have lived in MN and WI my whole life and salt in the winter is a definite issue. Here’s what I’ve been led to believe from experience and from what I’ve been told:
Salt will do more damage when wet and warm. Take road conditions and temp into consideration when considering a wash. I’ve also been led to believe that parking outdoors does have its advantages. Especially if you have a heated garage, that heat can accelerate salt’s ability to rust. I’ve also heard that even in non-heated garages, the air temp can increase when a warm vehicle is parked enough to make a difference, but it’s hard to say on that one. I will say that my grandpa’s old Olds that was garage-parked rusted far faster than any of the cars my dad and I have always left out in the winter. Some of this may depend on how long you wish to keep your car. Some people may not care if their habits lead to it rusting faster since they’ll be trading it in before body rust is a real issue anyhow. I’m not in that camp, though. Another point to consider is that some vehicle designs lend themselves to certain rust issues in the long run if certain areas accumulate debris and moisture more than they should.
In conclusion, I think washing is a good thing in the winter overall. Be sure to get the undercarriage cleaned as that can be overlooked at times.
Remember, rust can only be contained, not stopped. As the Neil Young song says, “Rust never sleeps,” unless you’re putting a car in a perfectly controlled environment. Most people can’t do that.