Proper tire pressure or not you will get squared tires in the cold. One day when it is -30 outside, go check the pressure, make sure it is dead on, then start it up and see if you have square tires or not.
Radial tires these days are softer than the bias-ply tires so the square tires don't last as long but you will still get them. Winter tires have even softer compound and again they still get them. If the tires stayed perfectly round and did not get that flat spot, well it would be called rims and not tires, you would feel every crack in the road and the car would ride like crap. Now this may or may not have an effect on it but I know up here they use road salt which even at lower temps will leave moisture on the tires, not sure if that make the effect of square tires more pronounced or not.
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