To my knowledge Google makes the first choice. They have bots which choose based on the content of the hub. Somewhere I did hear that we can make choices however when looking in AdSense couldn't find out where. If this is really a concern keep looking in AdSense until you find out and write a hub about it.
I did learn ads will change based on the feedback of early readers. This is within the first hour or so. They will also change if a hub is read many times.
My advice is worry more about what you can easily control, ebay and Amazon let you choose what you want placed with a simple process. Even so the first time I tried to place what I wanted it took almost an hour.
It is more important to write to your audience. They are who will decide where you will rank on a search engine. Unless your on page one it doesn't matter what ads are on the page.
In the end it is about how much time you are willing to invest. My belief is the time would be better spent creating new content. Spend as much time as possible using Google AdWords because every keyword cluster you find there have ads related to them already on file. This is the simplest way to receive targeted ads from Google.