> so we should be washing off salt from our cars when they treat the roads for snow?

Kinda depends on how long you're gonna keep the car

If you have ever driven old cars in the Midwest, terrifying amounts of rust from road salt is definitely a Thing. Or at least it used to be decades ago, I assume it still is now.

Still is.

Washing the salt off will reduce rate of corrosion. Probably the most important to wash is the underneath. I suppose it is more important for cars that do not do that many miles/year as in this case the internal parts will last a long time.