When I take my daily walk through my subscribed webmaster forums I happen to see lots of questions about placements of Adsense ad units. Many ask for reviews, some want to know the best performing units and others want general guidelines. Most of the time someone posts the Adsense heatmaps that have been posted on the official Google Adsense blog or tell the person asking the question to use the large or medium rectangle unit and blend it into their design.