NONSENSE! Very few if any people have all 3 cables (C/L/R) the same length. They can all vary in length, the speed of current through the cable would render all but the most insane differences in lengths inaudible.
At audio frequencies, length (within reason) shouldn't matter. Now, the RCL network of a cable could have some small effect, but I doubt it will be audible. So just wire it up and don't worry about it.
Jwrobinson and Ericbee, I beg to differ with you. I have heard many a difference or two in my system with different lengths of cables. Can you honestly tell me that your left and right speaker cables are different lengths? I doubt they are that different.
If you use identical amps and similar speakers for all three front channels, then identical lengths is recommended. This is to insure that the amplifier loading is the same for all three. The length has nothing to do with the propagation time.