Hi JHZ,
Washing your car is probably the best way of keeping down the effects of road salt. As far as cold weather washing, I wouldn't worry about it at all. Where I live up here in Western Canada there is no option to the cold weather, and no predictable repreaves to it either. We all still wash our cars in -20*C (-4*F) or even colder!!? Washing your car at extreme tempuratures will not hurt anything. Here is a few key things to do after you wash your car at below freezing temps:
1. Wash only at indoor DIY type carwashes.
2. When finished, wipe dry all of your door and trunk rubber seals AND corresponding surfaces. (this way you will be able to get out of or into your car after!!

)
3. Some people go as far as squirting lock de-icer into all of the key-holes on the vehicle as well. WD-40 does a great job of this as well, just don't get it everywhere, or be careful to wipe up any drips.
4. When it approaches -40*C (almost the same F) if one HAS to wash their vehicle WD-40 wiped on the door rubbers will help after drying.
I usually only go as far as #1 & #2, but have heard of other people who have done #3/4 also.
Hope this helps.
__________________
'85 300SD 'Otto' new to us Sept. '07
'82 300SD 'Jurgen' (July '06 fan shaft broke, toasted rad, 3am in the morning 60km from home, walked 16km hitchhiking....

)
'70 SS Chevelle (finally back on my property, in the shop on stands)
"The thinking required to solve a problem cannot be the same thinking used to create it!"-Einstien