i believe salt speeds up the oxidation problem, also known as rust. and i would imagine that a good washing on warmer days would certainly be beneficial. sand sucks because its like getting hit with a sandblaster anytime you drive. its NEVER a bad idea to keep your car clean. unless of course if its below freezing.
Subies come with a rust inhibitor (I beleive its zinc) in the primer coating.
The STi in specific has no factory underbody coating to protect the underbody which has exposed & unpainted steel, most of which is galvanized (also rust resistant, but not proof).
If you have to drive your STi in the winter and are worried about rust then just go to your local dealer and have them do it for you or stop by a Ziebart. Otherwise be sure to rinse the car off well top & bottom after driving it anywhere you think road salt may have been used. Or if you can just simply avoid drcving the car at all udring snow days as I hve and will be doing.
Where i live they use lots of salt and sand. So no STI for me this winter. Got a 4runner for drving in that junk. Although compared to my sports cars(heavily modded Type R and RX-7) of previous years the STI would actually be able to handle the snow and slush with the proper tires. But salt and sand on this baby, yuck! She gets to stay safe and clean in a temperature maintained garage for 3-4 months. Peace!!
salt can reduces your coating, the shiny protective coating. so you might want to wash it off ASAP and whenever possible. you see old cars? like really old ones, the paint looks flat. b/c of the sun and/or the salt corrotion. besides, it looks dirty on the car anyway. so dont try to keep it as an antique. lol!