In general I operate under the belief that the manufacturer knows their product better than anyone else. However, if you're talking about a couple of feet why take the CHANCE it MIGHT affect the sound. Even when dealing with networked cables like Transparent and MIT where one of the network's job is to compensate for the length I'd still advise against it, though less strongly. I'm more anal about these kind of details than most, but I believe that doing lots of really small stuff right can add up to better sound for very little trouble or expense.

Yeah- ditto that. Different lengths are a bad idea no matter what the cable. You set up some issue with timing, impedance and how the amp and speakers interact.