Q: How can I control the placement of Start Screen tiles in Windows 8?
A: With the RTM of Windows 8, there's no way to control the tiles placed on the Start Screen through Group Policy or other means. It is not a capability native to the OS.
The best approach is to give users some basic training on how to customize options, so they can tweak the Start Screen to their exact needs. The goal of the Start Screen is for users to customize it to their needs, which is why I believe there is no control possible post-deployment.
It is possible to customize the Start Screen as part of your base image that gets deployed to your environment. Additionally, in the unattend answer file (unattend.xml) there is a section, Microsoft-Windows-Shell-Setup\StartTiles, that has some customization of the Start Screen as documented at TechNet but there is no way to customize post deployment.