Should the cold air return when running AC be to open to top return or the lower return?

Honestly, you won't really notice much difference either way. However, in theory, you would open the top and close the bottom, as hot air rises and cool air falls, so you would want to "suck" the cool air up to the top return. Believe it or not (in my opinion, and from experience) a good ceiling fan will make much more difference than heigth of return.

THANK YOU. Makes sense. It was just bugging me that I was told there was a air return setting for heat and one for AC.
I do have ceiling fans in nearly every room so I will keep them going. I appreciate your response!

You bet, stay cool.