I washed my car with the hose on Thursday when it was 34*F.
I have washed my car many times in weather below freezing, and I use warm-hot water to do so. I did this for 2 years with my Maxima, and I've done it 1-2 times per week on the Protege. I know many people that wash their cars that often or moreso, and sometimes when it's under 20*F. I have never noticed any type of damage.
Think of these scenarios:
In the summer, a person (we'll call him Homer Simpson) plans to take his car to the Springfield Car Wash. It was 90*F and sunny, and this car was sitting in a large open nuclear plant parking lot all day while Homer was busy sleeping in Sector 7G. The car's surface temperature is likely to be 120*F or more (a black car can reach upwards of 170*F!). The car wash sprays it with water that is ~50*F.
On a hot, sunny, muggy, summer afternoon, Homer's car is sitting in the parking lot of the nuke plant again. The car's paint is heating up to ~120*F again, and then the warm muggy air rises up in the unstable air mass. Huge cumulonimbus clouds quickly build, and begin dumping large amounts of cold water onto Homer's hot paint.
These situations happen every day. If these situations were damaging to the paint, there would be a huge number of cars with cracking paint. This isn't the case. IMO, you should be more worried about the corrosion from the road salt, acid rain damage, and those evil brush car washes! Stay away from those brushes!
If it reaches above freezing during the winter, and you have the motivation to wash your car, do it. If you aren't willing to wash the car during the winter, make sure to apply some wax or sealant to it in the fall. If you don't do either one, then I'll be waiting for the call to revive your paint.
Merry Christmas!
