It really depends upon the building. Where are the water pipes, and how much insulation? For example, in a trailer, the pipes are hanging under the trailer, and they will freeze even if your thermostat is set on 80. Older homes with poor or no insulation may have this problem with water pipes in exterior walls.
In a modern home with modern insulation, I see no reason why 55 degrees would not be sufficient. You can always turn off the water before leaving as extra insurance.
Miss the Pacific Northwest yet?