My basement return and supply I usualy keep full open in winter only if
I want to heat it more for usage otherwise I leave returns and supplys
closed, my reason is even though heat rises it costs me more as I clock
the furnace run times and 24 hr run times as much longer with returns
and supply open. In summer I dont want my basement cooled as it is cool
and would be below 70 with supplys open, I have a dehumidifier in the
basement and leave it partialy open just to circulate air. Now and
usualy it is just slightly open for air circulation, since my basement
is tight and all apliances use Pvc fresh air intake and exaust