My 75 Honda Civic, (my first car) had no AC, and the rear windows had the little pop out hinges, so it had awesome airflow, and I always had the windows down.
My 96 Impala SS had ice cold air, but still had great airflow with the fronts down and the back ones cracked.
My 65 caddy coup de ville was a convertible. Nuff said.
My current 2002 civic, if you roll both fronts down, anything light will blow out the window, the air circulates, and by that I mean it blows in a circle. I smoke, so ashes just whirlwind around inside of it.
But I do roll my side down all the time.