The first poster is absolutely correct that you should NOT fool with the intake so as to reduce it. Most systems are designed for barely-adequate intake, and many are installed with inadequate intake.
Instead, I would close the output vents in the non-problematic zones so as to increase air flow to the problematic zones. (Close the basement vents in the summer and open them in the winter).
That said, and assuming that you have the rare "problem" of too much intake, you want to INCREASE air circulation in the problem zones, not decrease it. Thus, if you do close an intake vent, it should be the opposite of what you suggested in your question: close the intake in non-problematic zones, not in the problematic zones. Closing the intakes in the hotter zones during the summer will just allow the hot air to stagnate in those zones. Instead, encourage the system to draw that hot air into the system, cool it, and then send it back into the room.
That is why putting your car AC on recirculate ends up cooling your car ten times faster. Instead of pushing fresh cool air from the outside up against the interior hot air and passively cooling the hot air, you circulate the hot air through the AC system, and actively cool that hot air so as to remove the heat more efficiently.
If you close the intake in the problem zones, it will just take longer for those zones to cool/heat up. THAT is what really reduces efficiency.
As another poster advised, you do also want to be mindful of the stagnating effect of closing the intakes in the non-problematic zones. Again, I would not mess with it and would instead adjust the output vents (which would not cause the same stagnation problems because the system would still draw air in from the basement and send it up to the rest of the house).
[This message has been edited by Lawrence (edited July 23, 2001).]