Specifically, is it a good idea to use the "being greedy when others are fearful and fearful when others are greedy" heuristic?

I would say "yes, but." Really, the value comes from predicting a change before anyone else does it--saying "ah, the greed has spread as far as it can, and so now the only possible place for us to go is back towards fear, and that will happen all at once" or "ah, the fear has spread as far as it can, and so now the only possible place for us to go is back towards greed." (In my experience, fear is more contagious than greed.)

But always disagreeing with the herd is reversed stupidity, not intelligence.