Argon-Co2 Mix Gas in the Winter?

Storage won't hurt it, but don't count on using it for welding without warming it up quite a bit.

I think there is a possibility that the CO2 may separate from the argon and be a liquid (under pressure) at that temp (-30ºC/-34ºF) And if you open the valve the CO2 that comes out may instantly turn to a soild (dry ice snow flakes). Can't be good for your regulator. IIRC Argon has its solid and liquid phases somewhere below -185ºC/-300ºF so any phase change due to temperature/pressure should not be a problem, as it doesn't get that cold even in Ontario.