Again which I stated above, the most important thing is to try it, and certain equipment will react certain ways to the load, impeadance, capacitance, inductance etc.. of a length of cable as the same cable different length will change these properties. I have relativley short speaker cables right now myself at 4.5 ft and they sound perfect, and actually better than my 6.5 ft do of the same brand just in the way they sounded better because they took very little burn in. However with my horn speakers the shorter sounded "tinny" or lean.. They were probably just too fast or too low of resistance to run on that specific horn system with that amp.
By the way above someone stated a test that someone should do to see how different a shorter vs. longer length take to burn in and sound good, Well I can tell you my 4.5 ft sounded good instantly vs. the 6.5 foot, but after each burned in for a couple months they pretty much equaled out,
I only use the 4.5 now due to its cleaner install and I have mono blocks only 2 ft from the speakers. But yes I say the longer the cable the longer it takes to settle down. Interconnects seem to be quite a different deal than speaker cables however, mostly anything from 1 meter to 3 meter I have found very hard pressed to say one is ultimatley any better, however interconnects definatley have bigger influence on the sound if you change them then do the speaker cables in my trials. I still say go a short as possible with power lines however, that is just a no brainer.
I am no expert just stating facts in my scenario's.