Randy, you can certainly have PMs responsible for sub-projects separate from the Project Manager responsible for the master project. You did mention the one caveat, but Project Server prevents this from happening. I might assume (at least in most cases) that it is the sub-projects that are opened and managed more often than the master project. If you want/need to open the master project and all sub-projects, you can pretty easily see if they are already checked out. In short, if you need to then give it a go.