If you dont have a stereo
system, then only
plug one side in and still have the flexiblity to use both if the need arises

I have even done crazy things with running two media players on my computer and having each panned to one side, so I have control of each seperately on the
mixer.
Anyways, you were talking about computer speakers, so those are most likey stereo and if you board has stereo outputs it'd be worth it to go with the stereo option so you can pan.