I agree with everything JohnK has said in his previous post. However, I would like to add what I was told about speaker cable lengths when I first got into hifi quite some years ago.
In regards to a two-channel setup, I was told that it is very good practice to make sure that your speaker cables are of equal lengths and to keep them as short as needed for your space. This is because of the resistance and strain placed on your amp to carry the current to the speakers. Your amp is forced to work overtime on one channel if that specific cable is significantly longer than the other one. After a while, it could be the cause of failure on the amp. Please noticed that I said "significiantly longer." I'm sure that a relatively slight difference in variation is fine, and that the cables don't need to match down to the exact centimeter.
With that in mind, for my five-channel setup, I've got the front l/r speaker cables the same length (about 6' each), the l/r rears the same length as each other (about 15' each) and then a short run of 4' to the center channel.
Hope this helps.
Kevin
