Placement of AdSense ads depends on whether you have a content site, blog or forum. Also the site layout like left sidebar, right sidebar or full width page decides the suitable ad sizes to be displayed in those pages. Besides using recommended ad sizes by Google, it is also important to place ads in appropriate places on each page of your site to increase the revenue from AdSense.
Type and Layout of Site
Whether it is a forum, blog or content site, each type of site has its own layout and theoretically deciding ad placement may not exactly suit all cases in practical. Nevertheless, most of the sites fall into a standard layout having header, sidebar, main content area and footer with minor modifications like right sidebar, left sidebar or full width page.
In general Google suggests placing ads for a home page and article page based on the layout.
Ad Placement Example Case 1
Ad Placement Example Case 2
Other Factors for Ad Placement