my two story colonial home 2300 to 2400 sq ft. used to have furnace and ac, now has furance and heat pump, has supply register on floor in each room, and opposite wall has two return vents, high and low, distance between each register and return vents more than 10 to 15 ft apart. i remember my builder told me open high close low during winter, and open low, close high during summer, now my installer gave me a lecture about proper air mix, told me to do the opposite.

is it just a personal preference, can leave both open as well?