My home has both upper and lower internal return vents in each major room. Any suggestions on when to close one or the other would be appreciated. For example, do I close the top vents in the winter to pull cooler air into the furnace, or do I close the bottom vent to better circulate the heated air in the house? The former makes more sense, but I'd like to hear from someone who knows more than I.

I think the theory is, use the upper vents in the summer to return the hot air from the ceiling to the A/C, and the lower ones in the winter to return cool air at the floor to the furnace.