On SEO-Scoop there's a post analysing the data that AOL released to the general public (yes, the data that wasn't quite as anonymous as they thought... just ask user 4417749, more commonly known as Thelma Arnold)
Total Searches:9,038,794
Total Clicks: 4,926,623
Click Rank1: 2,075,765
Click Rank2: 586,100 = 3.5x less
Click Rank3: 418,643 = 4.9x less
Click Rank4: 298,532 = 6.9x less
Click Rank5: 242,169 = 8.5x less
Click Rank6: 199,541 = 10.4x less
Click Rank7: 168,080 = 12.3x less
Click Rank8: 148,489 = 14.0x less
Click Rank9: 140,356 = 14.8x less
Click Rank10: 147,551 = 14.1x less
So this data shows that in general you want to be as high as you can... pretty obvious eh? Well, the other things to take into account are the actual cost to be that high, the return on that spend, and the objective of the campaign. If you don't care about the cost, and are trying to generate buzz, or establish a brand, then aim high. If you are a business trying to make money, then you want to aim for the position that financially makes sense for you, again, fairly obvious. Interesting to see the data directly from a major search site though, as this means that the data is fairly broad, not covering just a particular niche or industry, which should therefore make it more statistically valid than some other studies that have been released on this topic.