In Windows 8 tile apps work really different from Windows 7 and you technically do not have to close them, instead they go to a suspended state when the end-user switch between apps. This is the way it works moving forward, letting Windows decide what is the best way to optimize the system resources.
When user open or switch to a new tile app, the system gives about 5 seconds to save settings or perform any other type of cleanup work, and then the app will enter into a suspended mode. It is still consuming system memory, but is now utilizing any CPU cycles. In the case the system needs to free up resources, it will close-down the suspended apps to recuperate more system resource.
For instance, if you have 7 tile apps that are in a suspended state and the App History is configured to 3, then in case the system needs more resources, the 4 oldest apps will be compared between each other to determine which one of them is using the most resources and then it will be terminated. The only app(s) that are currently running are those that you are actively using.
However in Windows 8 you can still be able to close tile apps or other type of apps by using the new redesigned Task Manager in the same way you do now in Windows 7.
Resource Dev Center