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

Not unless your ears can detect a phase difference of about 3 nanoseconds per meter difference. 

This is almost as entertaining as a closely matched UFC bout.

I prefer same length, but I need to ask the OP: "Why NOT the same length?"

If you're running in-wall, got it.  But if not in-wall, you've got plenty of space in your listening room if you have to drape/coil/hang/etc. (there goes another discussion...) the excess on the longer side.

Having one shorter than the other would be better than splicing to make them the same length.

if the right/left length differential is greater than 5% it will ruin the stereo imaging

if greater than 15% it will void the warranty on your speakers

if greater than 25% it will be may cause a short circuit in your power amp

if greater than 40% it will be a fire risk and you should only listen to music with nomex suit on and fire extinguisher in hand

😂

I'm gonna have to go with the OCD thing too, in the house. Funny thing I've done a lot of auto/equipment installs for stereo gear. I use to cut and trim all the extra wire. Nothing was the same length.. Butt connectors everywhere then shrink and zip tie everything.

5,000 watts in an excavator cab.. Like the guy gave a crap. Using a 10,000 pound breaker, breaking concrete with the volume at 10. Have to have ballistic glass just to keep from blowing it out..

I'm not a robot