The idea makes intuitive sense: close the vents in the guest room and the office, and the AC will push more air into the rooms you actually use. Less space to cool, less energy used.
It does not work that way.
What actually happens
Your AC blower is designed to move a fixed volume of air. Closing vents does not reduce the amount of air the blower moves — it increases the pressure inside the duct system. The blower pushes the same air against more resistance.
Duct leaks open up. Residential ductwork is not airtight. It has joints, seams, and connections that hold up under normal pressure but leak under higher pressure. When you close three or four vents, the added pressure pushes air out through every weak point in the duct system — into the attic, into wall cavities, into spaces you are not trying to cool. You are now paying to air-condition your attic.
Coil can freeze. Less air flowing across the evaporator coil means the coil temperature drops. Close enough vents and the coil drops below freezing. Ice forms. The system stops cooling and eventually shuts down.
Blower motor works harder. Pushing against higher pressure draws more current. The motor runs hotter and wears faster. Over time, this shortens the motor's lifespan.
The closed rooms get worse. A closed vent does not seal the room. Return air still pulls from the closed room (through gaps under the door, through the return duct if there is one). The room gets hot because conditioned air is not entering it, but the system is still pulling hot air from it and mixing it with the air going back to the coil. Overall system efficiency drops.
What to do instead
Keep all vents open. If specific rooms are consistently too warm or too cold, the issue is airflow balancing, duct sizing, or insulation — not vent position. A technician can measure airflow at each register and adjust dampers in the trunk line to distribute air more evenly.
If you want zone control — different temperatures in different parts of the house — that is a real solution, but it requires either a zoned duct system with motorized dampers or a ductless mini split for the independent zone. Closing supply vents is not zone control. It is system abuse.