I used to have a well ventilated space between my back and pack. I used an MSR frame pack (frame like an external Jan Sport Frame). When my back would get wet from sweat (as it always does) I would get cool. I didn't like the air getting to my back.

I now use a pack (home made) without a ventilated back and I prefer it. The pack bag rests directly against my skin. It keeps me warm when I want it to. When I want a shot of cool air to cool me off I just let the pack tip back a little to let air in.

I think this venting issue is a personal one with no right or wrong answer. You'll have to experiment and see what works for you.