Two months ago a google rep speak with me over the phone and they told me to make sure my rectangle ad loaded first. It wasn't because it was the second ad of three ads on the page. A month ago I took the advice and made the ad the first to be loaded by dropping the first ad I had below it. So now I have a rectangle showing first and then two horizontal ads lower down the page. My concern is that the exact opposite happened to what I was told. The rectangle ad CPC is now less than half of what it was. Although the CTR has increased. What is really bizarre is that the two horizontal ads that are further down the page quite a bit are averaging triple the CPC.
So how in the world can my best placed and first loaded ad be doing so poorly compared to the 2nd and 3rd ads which are not placed as well? Why would this change of making the rectangle ad first loaded drop the CPC in half?
I take issue with G's recommendation for skyscraper ads (not sure what "rectangle" ad is being referred to above but I find skyscrapers to be one of the most troublesome ad formats to work with in general). I often see them regularly out-performed by smaller horizontal units and units loaded after and before them in the html. Their text formatting has also been an issue of contention for me (ugly not that others aren't as well). I'm generally convinced that the horizontal leaderboard is still king but you'd be amazed at what simple link units can achieve or even mobile sized ads placed appropriately. As for progression in page flow and all these other recommendations G makes, I'd love to see them put their stats where their mouth is because the majority of recommendations are just hogwash (designed to benefit advertises more than publishers) and the best advice I've heard from anyone is to just build your site for a pleasant user experience first and foremost. That's really the only Google recommendation I follow regarding ad placement.
Be careful about focusing on CPC or CTR by themselves. What is important is the overall RPM of the ad location (and the page as a whole).
You generally want your best performing ad location to be the first location that is filled with an ad -- this does not mean that it needs to be highest up on the page. You can do clever things with CSS.