The better insulated your home is the more you can benefit from a programmable thermostat, uninsulated or poorly insulated homes will loose too much heat when the unit is turned down for an extended period of time and the amount of heat needed to bring the temperature up again negates the whole idea behind lowering the temperature setting.
Do not set thermostat below 70 degrees, especially in mild weather. The indoor coil will freeze up after approximately two hours, resulting in no further cooling. Restaurants, bars, and businesses usually have special switches to cycle the system, allowing lower temperature settings.
Set thermostat no lower than 68 degrees, leaving the thermostat set at any temperature between 68 and 74 degrees costs no more than leaving it at 65 degrees.
Note: Leave the system fan set to "on or continuous" setting all year round. This saves money and prevents hot and cold spots in the house.