How important is it to have identical speaker cable length?

To the speaker, no, a small difference is not a big deal.  But holmz is right, the additional pFd and uH could make a difference to your amplifier.  Remember, the output is also the input due to the feedback network.  As long as your cables are low in inductance and not too high in capacitance, you should be okay.   On the other hand, I have had good amplifiers tendency to oscillate when the capacitance in the cable is too high..

"This forum…..first answer:  absolutely no problem.   Second answer: oh my god no. Third answer: perhaps yes perhaps no.  Unbelievable."

I think it depends on the education of the poster.   When I got out of college I had a EE degree in analog electronic design.  I thought I knew a lot.   Then some time later I went back and got another engineer degree when I realized there was a lot I didn't know.   Now with over 40 years of analog design, I have come to the conclusion that the more you know, the more you realize how much you don't know.

I do agree with the same length concept.   Even if you don't sell them later, odds are you will change your system and have useless cables.

Robert