I think turning the thermostat down shouldn't cost - and will generally save - unless you have a heat pump that uses auxiliary heat coils or some other two stage system where the boost is inefficient.

Also, if your house is so well insulated that it never actually reaches the lower setpoint before you turn it back up, I suspect it would be a wash and wouldn't bother.

(Thermo was a long time ago, so I'm probably be misremembering.)

-jk