Wash it when you want to. Washing may wash some salt out, but then it might just get the salt wet (Dry salt does not cause rust).
When the temperature is cold enough to keep the snow snow and not met it, the salt is not causing any problems wet or dry.
Let's face it is going to get salt on it faster than you can take it off anyway. Modern cars are well protected by design and don't have the problems the cars did when I was a kid.