Douglas Self, in his writings on amplifiers in Electronics World ten years ago has discussed this in some detail. He has found that one gets a quciker, and more dependable response, mounting the bias transistor one of the output transistors, than on the heat sink, as the heat sink has a temperature lag compared to the transistor itself. I have tried it on his amp, and it works extremely well. I would think that commercial builders would stay away from doing it because it involves hand work, rather than the quicker and more economical robotic assembly. Try it--you will like it.