No, Mark, speaker cable lengths don't matter at all. The electrical signal takes about 1 nanosecond to travel over one feet of copper cable. Just in comparison, the sound wave takes as much as about 1 millisecond per feet in air -- Six orders of magnitude slower. So, before you get anal about 10 feet of cable length difference, you have to be careful NOT to move your head by more than 0.0001 inch while listening to music.
Incidentally, I am HIGHLY skeptical of the sonic benefits of bi-amping. In usual bi-amping configuration, you are still driving each power amp with a full-range audio signal -- in other words, each power amp channel is not "relieved" of its duties in any way under this situation. Everything else being equal, you will get much higher max available power by using the same two channels of power amp in a "bridged" configuration (the power amp has to be designed bridgeable, though).
In other words, you are basically "wasting" the power amp channels by investing them for a bi-amped, rather than bridged, configuration. In this sense, bi-amping is an audiophile myth, IMO.
It would be a different story if you entirely bypass the passive crossover circuits within the speakers, and replace them with an active crossover inserted
before the power amps -- the only configuration that I consider as a "real" multi-amping. But this is much more involved, since you have to design the active crossover parameters essentially from scratch. You have to know exactly what you are doing. And even in these configurations, the sonic benefits would be relatively small, if any.
