Lynx - in my case the problem was nothing to do with timing, it was an impedance issue. Cheap cable can have an unhealthily high impedance. Using long lengths of it limits the amount of current that the amp can drive through the cable and the speaker. So unequal lengths make one speaker quieter than the other one.
I'm not really persuaded that cable length will have a significant effect on timing. An electrical signal passes through metal wire at a significant fraction of the speed of light (in air), so you're talking hundreds of thousands of times faster than sound travels though the air from the speaker to the listener. It would take rather less than a microsecond for a signal to travel through a cable 100 metres long. By contrast sound would take nearly a third of a second to travel that far.
What might also be an issue (I'd have to think about this some more to be sure) is that cable is not perfectly "transparent" to an electrical signal: the impedance will vary, depending on signal frequency (the capacitive and inductive properties of the wire) and perhaps also amplitude. For example, some cable might transmit higher frequencies better than lower ones, or vice versa. If you have a longer cable then the influence of the sound on the signal may be more significant than it would be for a shorter one, which means the two signals coming out of the speakers will end up with subtly different tonal properties.