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

No, it's a non-issue.  I defy anyone to hear a difference with the lengths used in the average domestic room. There are more phase issues and time smear from a speaker crossover and even more if the speaker is ported. As far as unbalancing the image, room acoustics will make a farce of that.

If the longer cable is a gauge heavier then even the amp's damping factor is maintained.  Keep the cable to the nearest speaker short. Consider the alternative. If there is, say, 8ft extra, what will you do? Coiling it up is worse than having different lengths. Think inductor tucked away amongst all the other mess of wires.

Rather pay attention to your room' acoustics which will do more, much more, than fussing over equal lengths.

Mitsy says: "Are you kidding? Of course not!":

 

And, Mitsy can hear up to 45Khz. 

Hey Christian:  Hi they are not the same length because one speaker is +/- 10 further from the equip. The stereo is in the corner of the room due to fireplace location, sliding doors, etc... Also the wire to that far speaker goes under the house so that adds a few feet as well.

Ah - so custom install, not quite full in-wall, but close.  Yes, no issues with differing lengths.  I've said before I have a tough time discerning differences between cables, but if say you have your audio rack on the outside edge of once of the speakers (which I thought was the case), why not buy an identical set, and just coil/drape/suspend/whatever the cables over to the speakers.  If anything, as said before - it would help with resale, but I figure you won't be reselling the underfloor wire anytime soon.  Sounds like you've got a fun project ahead! 😃

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.

Same here - I started doing my own car audio custom installs in college (were those the days!) - underseat/trunk/dash mounted amps, and wires of way too varying lengths going all over the place.  It's probably the OCD in many of us when I got into the home stereo listening craze (er- wife calls it a disease): a pair a speakers, audio rack in the middle, and EQUAL length cables. 😋

Again, not sure if I could tell the difference if one side was longer than the other, but it wouldn't "sit" right with me if they were differing lengths. 

In the mean time… one highly recommended speaker cable to consider (let’s see if I can get this right); HD12-3? Years ago either Stereophile or the Absolute Sound did a comparison of a number of low to intermediate speaker cables. They threw in a Home Depot extension cord… ends cut off. It actually sounded pretty good… didn’t win against $1K cables but sounded way better than lamp cord and Radio Shack 18” gauge speaker wire”. I loved the article it was great fun and informative. While I use $5K Transparent cables on my main system. I use HD12-3 on my office system… hidden because of the orange and black color behind my desk.

Sounds like you have a mold problem, ghdprentice. Just kidding... but are you serious about using an extension cord? It’s about $80 for 100 feet on Amazon; for about $10 more, you can get nice 10 gauge real live speaker wire.