I agree that shutting 1 vent isn’t going to kill your system. However, that one room isn’t insulated on interior walls. Best to leave it alone for overall comfort. You aren’t saving $ closing the vent. The system is going to run until the thermostat is satisfied no matter what.

Also, best thing for the entire building is to run the fan constantly (I forgot to mention this in the original). Balances the building much better than just the entire system kicking on and off every 15 min or so.