When to 'force stop' and it's effects
The answer is easy: Under normal circumstances you never need to "force stop" any app.
Android automatically handles which app should be stopped in case the system is running out of free RAM.

Anyway even if you force an app to stop it usually should be able to start again at any time. Note that "starting an app" may even happen without user interaction based on certain events like change of battery level, a new Wifi was found, or simply because another app has triggered the app to run in background.