[siriusb]

It depends upon ur case fan arrangement too. For example, in my old cabinet, my temps were lower when I kept it open. But with my new cabinet which has fans on the side panel, temps rise insanely unless I put the cover on.
I find it better this way, coz I don't have to deal with tons of dust on my motherbord which seems to attract dust like hell. I guess it's the static electricity.