When water freezes, it expands. Water freezes at 32 degrees. How fast the water will freeze is a function of the amount of water (mass), the ambient temperature (the colder it is, the faster the water will freeze), the time water is exposed to freezing temperature (the longer that time the more likely water will freeze) and the wind chill (how fast the mass will reach the ambient temperature). Anytime a pipe with water in it is exposed to freezing temperatures, there is a risk of damage. What your risk tolerance is only you can answer.