The simple answer to this question is probably not. Heating and air conditioning systems are designed to pass a certain amount of air in order for the system to work properly. If you close one vent in a small bathroom you never use… you are probably ok. If you close too many vents it will back up air in your system and can shorten the life of your equipment . You are not saving any money by closing off vents. If you have air balance issues a qualified technician should take a look and recommend modifications if necessary.