We cover our ac vents with flat magnetic stuff so as to conserve in places like the guest room.
You’re creating back pressure on your unit which you don’t want unless they have relief valves which puts the treated air in the attic which you don’t want. That’s a lose lose situation.
The problem with that is the design of the systems. Central A/C systems are designed for the flow achieved when all the vents are open. Tamping down vents can cause problems. Although there are some modern high end systems with variable air flow/compressor speed.