Well, it depends on whether you're actually going to be hibernating the system. When you hibernate the system, all the contents of RAM get written to a file on your HD, then your machine turns off. This means that when you boot the system back up, you're in exactly the same state as when you shut it down, and you boot faster than if you had to wait for everything to reload. The downside is that you need another file on your HD that is the size of your physical RAM; the file is there taking up space as long as you have the hibernation feature enabled. However, if you don't plan on using hibernation, there's no point in turning it on and wasting space.