If you're using a cable that's so resistive or (with speakers where it can matter) so reactive that an extra 12 feet causes an audible difference in sound, my advice would be to change cables. But if you are intentionally using your cables as a tone control, then yes they have to be the same length.
The timing difference really is too small to matter. Think about it: You can also create a timing difference by moving one speaker slightly closer to you than the other. Adding an extra 12 feet of wire to one channel would have the same timing impact as moving a speaker a few millimeters. (I'm estimating here, but I'm pretty sure that's the right order of magnitude.) Now, did you use a tape measure that accurate when you placed your speakers? And what happens when you move your head? If this mattered, audio would sound very weird.
The timing difference really is too small to matter. Think about it: You can also create a timing difference by moving one speaker slightly closer to you than the other. Adding an extra 12 feet of wire to one channel would have the same timing impact as moving a speaker a few millimeters. (I'm estimating here, but I'm pretty sure that's the right order of magnitude.) Now, did you use a tape measure that accurate when you placed your speakers? And what happens when you move your head? If this mattered, audio would sound very weird.