Depends on how often you access them. It's actually more energy costly to end the process for apps than it is to keep them cached and in the background. The starting up process and calling of all instances/APIs is resource guzzling. For apps that you frequent (minus Facebook if you use it because it's a resource guzzler regardless), you won't need to close them out completely since in the long run, it uses more battery life than keeping it in the background, that is unless you have background app refresher on, in which case, it will drain regardless.