Most background apps in IOS are not running but are 'paused' with a completely saved state. Only a few apps such as music players, GPS apps and few others are capable of actually multitasking in the background.
Also because the background tasks are in a 'saved' state, they are not consuming any CPU or memory to run, IOS is smart enough to give more memory from those background apps to your running on-screen task. For this reason you don't really need to close those apps down ever as memory management is fully automatic.
Occasionally, I do close out my applications or reboot the device - but Apple does not recommend doing that for no reason.