By hibernating an app, what you're doing is basically putting it out of service while you're not actively using it. In other words, hibernated apps will be completely static (and not consuming battery power) as long as you don’t actively start them and bring them to the foreground.
Because of this, apps such as alarms or instant messaging services should never be hibernated. Other apps you're better off hibernating because their functions when in the background don’t really provide you with anything except battery consumption.
Greenify is a very useful app that will allow you to save a quite a bit of battery power. Plus, unlike other apps that let you 'kill off' apps, with Greenify you can easily and quickly go back to using them by simply starting them up.