I have been doing heating for 40 years and this is how i engineer a house.
First take a heat loss on every room of the home. This tells me what size furnace i need. Furnaces come with different blowers and speeds. So to get the right blower i need, we use a rule that we find out the cubic of air in each room, and all the air in the home should be able to go through the heating and cooling unit 5 times per hr. so we can properly treat the air. So now we convert heat needed in each room into CFM of needed in each room. From that we lay out our duct system.
We can balance the system, but people have different wants and needs.
You need to move enough air through the furnace so you don't over heat it. You also need to move enough air across the cooling coil so it wont freeze up, But normally turning off 2 or 3 hot air reg. will not effect you system that much. It will just up the static pressure and move a little air out the other open. If you go into a bedroom and shut the door and close the hot air open, when you furnace comes it will pull a fast vacuum on the room and then you will not get any more air flow out of the room.
So in a base in the summer most people will close some of the opening.
In a upper story you have to close the opening down some in the winter, and open them back up in the summer time to cool it. Later Paul