I have never gotten "stressed" over low fuel temperature in the wings as fuel is continually circulated by pumps and warmed by the oil/heat exchanger prior to injection into the combustion chamber. But I know that the freezing point for Jet A begins at -37C, and for Jet A1 at -40C. These are threshold temperatures where crystals begin to form, but it doesn't mean that if you're cruising for 9 hours at a Flight Level at -56C ambient temperature that all the fuel in the wings will turn into a solid. It's just as if you were to take a blender full of water into freezing conditions; the water will slowly begin to freeze at 0C, but if you were to run the motor on the blender the water won't begin to gradually freeze until it's at a much lower temperature.