There is absolutely no reason to cool rooms that aren’t being used. The more space that needs to be cooled, the harder your air conditioner will have to work. Since most people spend the vast majority of their time in just a few rooms, make sure the cooling vents in the other rooms are closed.
One exception is if you have a room with a temperature sensor in it that can’t be turned off. In this case, closing the vent will make your air conditioner work harder since the sensor will always think it isn’t cool enough.
All you need is about ten minutes of time to walk around your house and locate the air conditioning vents in each room. Check closets too. You’d be surprised where vents are in some houses, especially older ones. When you locate a vent located in a rarely-used room, make sure that the vent is closed tight. By doing so you’ll make your air conditioner work less, which will put extra money into your pocket instead of handing it over to the electricity company.