This is one of the items that is tested in your Windows 8 App when you use the WACK.
If you are in Southern California, please email this post for more information on getting a token for your app by June 30, 2012!
Through background tasks, Metro style apps can run while suspended or terminated. Despite this flexible option being available, not all applications need background tasks.
Review your scenario carefully and decide if you must run in the background, how-often, and what would happen if the user – who controls the lock screen apps – does not approve for your app to run in the background.
To become proficient on background tasks, check the introduction to background tasks white paper and then design your application for the right usage.
If you do decide to declare your application for a background task, ensure that you have the right assets (for example, badges for lock screen) and that you have the right triggers and conditions so your app runs when needed and within the allotted slots for background tasks.