While -56°C is the 'normal' temperature that the standard atmosphere model predicts, there are variations, just as on ground. -66°C is quite low, but not unheard of. The Airbus A320 is certified to fly at teperatures as low as -70°C, although even in warmer cruizing levels, fuel temperature might become an issue.
Over the north pole, the atmosphere at 30000 ft might be surprisingly warm, the reason being that the tropopause is lower (that's where the air doesn't get colder anymore as you go up).