Wow, waited a whole eight minutes for a response...patient aren't we.
Anyway, there is no difference between tapping into the speaker wires right behind the HU or doing it right at the rear speakers, it's the same wire either way. Tapping at the speakers will be easier, though because you won't have to try to run RCAs all the way through the car (assuming that you are just adding a sub, other wise you will have to run wiring to the front for speakers and where you tap won't matter at all.)