The back button should take you out of the app (when you've backed out of any nested pages that might have been created). A this point the app is stopped; at worst, the app will set up a background agent which will either work at full tilt for 10 minutes, or will make a brief poll every half hour. Either way you're not going to drain your battery while you sit at work.
If you go to the Phone settings screen and swipe over to 'applications' the first entry is 'background tasks' where you can check what's running in the background.
Thank you, Dr. Herbie, good info here.