Some years ago I was obsessive about my filtering ads and guessing which ones paid low and which paid high (I have extensive experience in AdWords which does give me a bit of extra knowledge as to some niches) as well as worrying over other aspects, and you know what, no matter what I did, the earnings were essentially the same. Maybe little rises, but nothing huge. So I have given it all up. The only time I'll block an ad now is if it's very obviously inappropriate for the site, or not working (I've had issues with a couple of rich media ads that just display as blank with nothing but the blue arrow)
This past year I pretty much enabled everything to run everywhere, and while my CTR is down, my EPC is more than double what it's been in any previous year.
The issues around my site, and placements and color - those are things I do worry about and obsess over, but Google does know more about serving ads than I do, and the less I try to tweak the dials, the more money I make and the less stress I have.
So yea, in my experience, it's a myth.