I have a friend who has been driving for over 45 years and still owns "almost" every car, motorcycle, tractor, truck he ever purchased..and they are all rust free. He has more large garages than I have vehicles and swears he has put less into his vehicles than I the last 40 years. He lives on a dirt road in the North East salt belt and commuted 50 miles per day in this area in 20-25 year old VW Beetles and Rabbits over the 25+ years we worked together. I trust his judgment on vehicle maintenance. He:
1 keeps them garaged or under cover when not used winter or summer.
2 Rinses them after each use if dirt or salt were present while driving.
3 Keeps newspaper or water proof barrier between shoes and floormats
4 waxes cars he uses usually just once or twice a year depending upon use
5 starts and runs all vehicles (40 +) regularly
6 discourages regular driving by others in use of vehicles in family except by trusted son other then their own
7 rust preventative treatments with oil (the procedure I've mentioned here) every one to two years depending upon use frequency
8 regular mechanical service by owners manual recommendation (usually by himself)
He is an environmentalist and feels he is more so with his routines than letting his cars rot and replacing them more frequently. Even his tractors look "show" an hour after hard use. BTW, he treats friends and family with the same care and thoughtfulness.
My feeling on your question. Water does collect at the bottoms of metal seams which even drain holes can't take care of completely. Rain and pollution will collect as well as road salt so diluting with fresh water is worthwhile. Limit oxygen to prevent rust formation by "sealing" with oil treatments. Anything else more substantial will dry out over time, crack and collect and retain moisture causing more corrosion. As far as leaving outdoors, you have to way the damages that can occur from each. Summer sun is a huge culprit to car's finish and seals, worse than in winter. Rust prevention, if you keep a car that long, is a maintenance item, not just an avoidance item.
Contrary to what you will hear by some; "all cars are rusting". Manufacturers just do a better job of delay and or hiding that fact of late. Check your recent recalls...