Don't take any chances, unless someone is going to check on the house and start a generator when the power goes out. Drain the system and put in antifreeze, the cost will be offset by the savings and if you have forced hot water heat the antifreeze will stay in the system and winterizing the next season will cost even less. I have friends who had pipes burst two years in a row because the turned the heat down and then when the power went out for more than two days they had a lot of damage. Insurance may cover the cost but not the aggravation.