I don't know how often my car was wintered before I bought it but it's currently rust free.
I wintered it one year but have been storing it for the last 2 years and will continue to.
Anyway, if for some reason it get a little salty in the fall or spring, I just make a point of power washing the whole underside. Anytime it's on the hoist I check all the common rust areas. When it comes to chipped or scraped paint underneath, I just give it a spray with some of the undercoating from Canadian tire.
I don't believe in rust proofing. It's just makes a big sticky mess that collects dirt.