I think you will find that, yes, the rebound piston just moving throught the fluid will create some damping in both directions, even though the ports releive the majority of this. It takes some resistance to push anything through a fluid, whether it be a ported piston, or a peice of window screen. ;)
The rebound adjuster (clicker) on the other hand definitely effects damping in both directions as it is a direct bleed, metered only by the adjuster needle, with no check valve. The effect is not huge, but definitely present.