I didn't want to paint them, so I got black poster board and cut it so it slides in between the glass and the plastic trim at the top and bottom. You can try it out for $2, if you don't like it, take it off. If you do like it, then think about painting.
I've never found it necessary either. But I understand his point, and something to consider if you're keeping particularly shy fish. I paint the sides of my fishroom tanks that are right next to each other, but that's all.
I think it would have helped my completely spastic xenos, but I didn't think of it when I had them. But for all my other tangs (altos, cyps, brichardi,julies, caudos, paracyps, gobies, shellies...) they don't care one bit and will watch me turn the corner hoping I'll stop to feed them.
I've got two sides of all the tank in my fishroom painted to block the view from one tank to another. For tanks outside the fishroom, I think it's a good idea to paint them only if they're in the corner.
Taking paint off a tank with a razor blade is easier that putting it on.
I'm not sure it makes a difference with individual tanks placed around the house. Most fish seemed to become rather accustomed to seeing movement in the room and as triscuit pointed out, they usually respond excitedly in hopeful anticipation of some food. Having said that, I have resorted to painting the back and sides of all of the tanks in my fishroom with blue Krylon spray paint from ACE Hardware. It goes on easy and dries to the touch in 10 minutes. My fishroom tanks are on racks and are placed to allow the maximum number of tanks on each shelf. As such, I may have adult breeders right next to a grow-out tank of young fry. I can speak from experience when I say that the fry will really appreciate it if they don't have to watch a pair of hungry breeders drool (at least I think fish can drool) excitedly from the next tank while they swim around. It also allows the breeders some "privacy" when spawning, rather than being distracted by another pair of breeders right next door. And if nothing else, it hides all the cords and hoses behind the tanks and makes a great background for photographing your fish!