Most thermostats don't go much lower than 55 degrees but, if the furnace and thermostat are working well togather, nothing should freeze. The furnace should maintain 55 and water doesn't freeze until 32.
A few tips to go along with the 55:
keep cabinet and vanity doors open
keep all interior doors open for full air movement
Keep the fan on, at a low speed, to keep the air mixed and regulate the temp.
If you are leaving the house for a while, turn the main water supply off; don't forget to turn the water heater off too. You could leave the pilot light running on a gas water heater. It is not that I think the water lines would freeze at the 55 but there could be a power failure and the thermostat would not have anything to say about it.