You should never close more than one register in your home if the air conditioning system was installed by ACT. If your duct system was not installed by ACT, you should not close any registers. Closing off registers does not save energy! Find out about a temperature zoning system. If you shut off air to rooms, they still will have heat gain from the outside. This heat gain is transfers to the rest of the house through doors, walls, ceilings, etc. This heat gain will cause your unit to work just as hard as if all the registers were open. There are no energy savings to be gained from closing off registers.