<< What I mean is my current soundcard isn't 4 channel. So I would not be able to even hook the rears up, right? >>
So? you might as well hook up the two speakers that you already can. Plus, on my Cambridge fps2000s, if you only connect the wire for the front speakers, it automatically mirrors the sound to the back. Maybe the z560s do this too. Like everyone else said, it can't exactly hurt, now can it?
Go for it! 😎