If I may add mga lodi, I am not an Android user but this works for both platform:
Never close any background app (like swiping up/side ways) unless its "unresponsive". Apps that appear to be running in the multitasking view are actually frozen and will use very little – if any – power.
Your battery life will be worse and it will take much longer to switch apps if you force quit apps in the background.
When you swipe-up or side ways to force quit an app, your phone needs to open software from scratch next time it’s launched.
That is more resource-intensive than simply resuming the same app from a frozen state.
Biggest battery drain are apps that continuously uses GPS to track location, email apps that automatically fetches data, anti-virus that runs and scans real time and tons of notifications. Other than that, its safe to just leave your unused app in the background.
Hope that helps ^_^