Well, it depends on the engine, to be honest. But you want real clicks not dodgy affiliates or competitors. I’ve found that just below the fold gets as many real clicks as just above it from my own tests and it requires some effort to view it. Though you might like to try just above the fold or, in the case of GoClick, the 6th spot which is both (the title is above the fold, the description below it). I just like to try to minimise the potential rubbish if I’m bidding on the really big terms.
Obviously if you’re bidding on something like ‘high roller casinos’ (picking one completely out of the air 
) then you will need to be above the fold as all the competing listings will be about the same thing.