Do speaker wires have to be the same length for each speaker?

For a given material, resistance and length formula clearly speaks that the resistance is directly proportional to its length.
R∝L
Which implies that-

When the length of the material is increased, its value of resistance also increases.
When the length of the material decreases, its value of resistance will also decrease.

"Relation Between Resistance And Length - At BYJU’S" https://byjus.com/physics/relation-between-resistance-and-length/

It make not make much of a difference at the lengths we are talking about, but different length cables will have different resistance.

As P=I *V (power equals current times voltage) and I  = V/R, P=V^2 / R, the more resistance, the less power. So different lengths will make a difference. It may not be noticeable, but different it is. 

Yes, or YOU WILL BE ARRESTED.

Send me your address IMMEDIATELY so we can measure your wires.  Better get a good lawyer and bail bondsman, pal.

Right Christian, listen to the music!! Last night Dire Straits & Anita Baker, both sound beautiful enough for me on my little vintage system!

p.s. richopp, I moved and changed my address 😀

No.