As a rule of thumb (according to several experts), whenever you got a sub woofer in your system, just set ALL speakers to SMALL and you'll be done. That really gets the burden out from your speakers & receiver and sends the heavy loads down to the sub, which has the only function of getting huge quantities of bass/LFE signals.
In other hand, if you DON'T have a sub and your main speakers are really FULL RANGE, than is a different story; even though some experts still recommends you to set the full range speakers to small (if you have a sub).
Roughly talking, we could consider this something like giving an "extra breath" to both, your receiver & speakers, meaning a longer endurance, though.
Cheers / Avliner.