[olddog]

Assuming that closing off one room does not alter total air flow through the furnace to the point that it affects the furnaces effeciency, then it comes down to a simple question. Does heating less square feet of house reduce engergy costs? Hell yes it does. Proof. If you add on a 20 x 20 room to your house, do you think it will impact your heat bill? Yes, and it will cost you more.

A well installed system will have enough registers and big enough duct work that closing one register will have little impact on the furnace effeciency. Not all systems will be installed well and your experiance may vary.

A programable thermostat that backs the heat setpoint off when your not at home and at night made a big difference for me.