As our own Sebatien recently explained, force-quitting multitasking apps to save your iPhone's battery life is a terrible idea because A) Apple is giving developers proper tools to have their apps work efficiently in the background; and B) users should trust the operating system is effectively managing apps that aren’t active.
Today, Apple has confirmed that users should not force-quit apps frequently and has said that doing so won't help extend the iPhone's battery life at all.