Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
unless you're using external drives, odds are it will do exactly nothing in normal usage whether on or off.
 
Not sure if it also puts SSDs into a deeper idle sleep or something. If it's on by default, I would leave it alone.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.