its not good to block off vents. This causes back ups in the duct system and actually the furnace blower is blowing out the same amount of air, the ducts are off and the air gets all jamned up and is congested in getting out the few openings available, so the unit is under a load.
Then you get all the full blast of air to the rooms left open. Thus, you are still cooking the same amount of air and its still costing the same because its being conditioned (heated or cooled).
I would leave everything open and lower the thermostat and maybe add some room heaters. Caution, room heaters are dangerous, there again,you could save money on heating bills but have medical bills or a fire on the other hand.