I agree with above, if you're talking about at dealers they're probably twiddling their thumbs right now! It might take a while to get a good look at the outside of the car, but they'll clean it up and you can drive it and check out the inside and so forth. Doesn't matter if the car has salt on it, still drives the same. They can wash it after the drive perhaps so you can check the condition of the exterior.
If you're talking private party, it'll be trickier. You might need to spring for a car wash during the drive in order to see it properly.
Either way, I agree it would be good to test drive again in better weather if possible. But usually that will on,y improve the ride. The bad weather gives you a perspective you don't often gets test drives. I'd go ahead and start shopping in the bad weather though.