I use a plastic garbage bag or two. I want to minimize salt on my metal edges and the edges of others. It's really noticeable on long drives. Your edges without cover will be rusty. If it's a really short drive I won't bother. Anything over 30min. or roads that will be really salty and I cover.
Salt and water accelerates corrosion. If you don't want it on long trips, many short trips will do the same damage.
How about an el cheapo rayon bag that you can toss in the washing machine when it gets gross?
Read on another forum: "If someone held a gun to my head and said, "You have to move to Salida tomorrow", I'd probably do it. If they told me I had to go to Breckenridge instead, I think I'd just let them pull the trigger."
I wouldn't worry too much about protecting your gear with a bag while it's on the rack. The lift lines are WAY more abusive to your board than it being out in the open air while driving to/from the hill. If you ride park at all, then you should view your equipment as relatively disposable. If you're getting more than a season or two out of your board, then I imagine you're riding less than 20 days a year.
As for road salt, I just rinse my boards off when I get home, or at least wipe down the base and edges.