If the roads are really "salty" I'll run it through the automatic with undercarriage blaster every couple weeks.(No brushes, just pressure)
+1, although I tend to go to the coin-op much more frequently than the touchless because its much cheaper and I can do a better job on the wheels/wheel wells than the touchless does.
If the roads are sloppy, then I just leave it dirty, no point in getting it "clean" if its not going to stay that way for more than a day.