If you don't provide adequate cover for your fish within the tanks (caves, logs, plants), and notice them ebing skittish, it might be worth it to cover three sides, or at least the back. My tank is a jungle, with lots of places to hide, and I've never noticed my fish being stressed by this.
Val--fish don't use thier eyes to detect the glass--they can sense it using their lateral line. They've documented blind fish have no problem avoiding obstacles such as glass.