You'll also want to close those out when you are done using them as they are still running in the background.
Most of what Apple has implemented is not multitasking but rather task switching.
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.