The underside of my car is rust free, but when I wash it, I make sure I get well underneath the usual areas with the washer etc.
Its best to wash winter grime especially slat and grit off asap. On motorbikes you can see all the bolts etc start to rust, so in winter I cover mine with grease and keep hosing it down after each ride.
As for cars, it will be the same, but their components are less expossed. Howver best to wash thoroughly as often as possible.
salt is bad for any car....my best guess is if you have to travel, soon as you are back or get the chance, powerhose with hot soap the underside of the car, i always do, but in theory the underside of the car was waxoiled , to stop the rusting..