Tough to put a fine point on this one, obviously...
Going through the process of the water change definitely stimulates my fish, especially cichlids - they seem to "play" in the water and will actively try and forage in any substrate that gets kicked up.
On the other hand, the only fish that seems disturbed in a bad way by my water changes would be any plecostomus in the tank - they seem to take longer to recover from the "trauma" and will change color and position in the tank all the while.
Voted 3.