Life is too short to prat about with bits of cardboard stuffed into radiators, also I am sure the aircon will be impressed at losing the cooling air to its condenser. My car works fine as it is. I have been in both Canada and Norway in winter with a temperature of minus lots, I don't recall anyone blocking off their radiator. It would make for an interesting warranty claim, "My engine has overheating damage because I blocked off the radiator". I can see the dealers staff in hysterics, after you have left of course!