This answer is tricky because A) there is a lot I don't know about your home and B) because there's a lot of math and I don't know much about your system design. But here is what I can tell you. If you shut down rooms that are on exterior walls, then the heat will penetrate further into the home. Cool rooms help rooms around them stay cool since they share walls. Make sense? Your system is to be designed so that each register allows a certain amount of air and a 6" pipe (for example) can only move so much air. (Trying to shove 5 lbs into a 2 lb box) You can only accomplish so much with this method, and if you overload the duct, you'll only create back pressure and not move enough air. Perhaps you should consider zoning your system. This would allow different areas of the home to basically open and close as demanded by the thermostats in those zones and not leaving the system to operate full time at an improper balance or you run up and down doing it manually. There is a logical way to do what you say, but I don't recommend running around shutting off all your dampers randomly or just saying "all the downstairs ones." Unfortunately, it's not something I can just teach you or that you'd even have the tools to do it. Tinkering won't hurt, but wholesale changes could make things work harder than they should or perhaps make the issue worse. Keep your fan running. It will help even out temperatures in the home, costs little, and cleans your air in the process.