All we can tell you is how it’s supposed to work on a properly designed house. How it actually works depends on the quality of the installation, quality of the insulation, placement of your furniture. Installation of drapes where they were not originally planned at the time of design. quality of the windows, quality of the vapor barrier, placement of your thermostat, weather or not rodents have degraded your vapor barrier and insulation, number of people in your house, equipment that you keep in each room,...the list can go on and on...
I would suggest doing what makes you feel most comfortable. Now you know how it is SUPPOSED to work on paper. Reality is always a different story. The problem is, a poor job done by a different trade can result in poor performance from your HVAC. If the insulation of insulation wasn’t preformed correct it’s effect would be quite obvious because now, for example, the ceiling may not have the correct thickness of insulation on it, or it may have a hole in the insulation, now causing heat losses and gains where they were not designed to be. The deflector may be a solution there. An electrician or a plumber may not have sealed up their holes that they drilled allowing for infiltration into a room. The result seems like the duct was undersized but the reality is it was sized right for the load but the additional load created by the other trades was never supposed to be there and that new load was not accounted for.
Everybody makes mistakes. Maybe it is best to have another register under a window that didn’t have one. Maybe it was in the plans but the builder didn’t want to add a required bulk head to make it possible so they said they would put a baseboard heater there instead, then never followed through with it. Owners never know when they buy a used house or quite often a new one, if all the plans were followed to a “T”
In reality, bedrooms turn into computer rooms or games rooms that include additional heat load from computers, TVs, stereo equipment, an unfortunate number of human bodies, and the room gets the complaint that it’s always too hot, the AC can’t keep up. But if you provide enough airflow to that room to keep it cool on those few days when company is over then every other day when it is not being used will be like a fridge. That can be planned for during design but after the fact it’s very hard to resolve that. Walls get moved. Walls get added. If you are walking into a used house you may inherit the problems that go along with those modifications. What used to be an open area is now enclosed with a very cold hallway because it no longer has heat eat supplied to it even though it runs the length of the house with a window at the end... this of course is all hypothetical, but these examples happen all the time.
Your heat load doesn’t change when you add deflectors, just your comfort level... (and ductwork static pressures, but that’s probably better left for a different thread)