I doubted my own remembrance that the Classic's/Classic II's fan was placed horizontally in the front bottom of the chassis, but when I just a moment ago undressed a Classic, there it was. Ventilation is patently 'positive', in that the fan is pushing air into the case from the bottom-most vent, as you wrote, and exhausting through the other five vents. Of course, the Classics would be amongst the few Macs to use positive ventilation rather than an exhaust system. Airflow from any of the escape vents is not even palpable in the case of the Classic II I have running beside an SE/30 at the moment. The SE/30's exhaust stream is quite forceful, by contrast.
Given all of the above, putting either Classic on anything but a hard flat surface should be a no-no, because it will potentially throttle the supply of cool input air. If macintoshman wishes to do low-level (groan ...) work with his Classic II, all he needs is a stiff 10" x 10" board to slip under it. However, the caveat about dust bunnies still holds.
de