It’s a rule of thumb in that you want the wires to roughly be the same length.
This is done to ensure that resistive losses remain approx equal so that one speaker doesn’t sound louder than the other.
It’s ok to have say 5 meters for L and 6 for R.
What you don’t want is say 1 meter for L and 25 m for R.
Having said that , any differences on account of unequal cable lengths will be very subtle and can always be corrected through the gain control on your AVR.
So if someone has already wired with unequal lengths and put the cables in a conduit, it would not be worthwhile to re-wire but for a fresh setup, you might as well keep ém equal
And no for the latter, you can keep your cable lengths as short as you want