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

Yes! And coiling the extra cable is NOT audible, don't care what anyone says. In fact, instead of coiling , you can fold it over onto itself and use ty wraps. If using a different length for each channel,  the resistance, inductance, and capacitance will differ for each channel. Why not eliminate that by using same length?

As an exercise for the perennially geeked, calculating the differences in capacitance, inductance should give a few seconds of mathematical bliss - at least.

intuitively, one would want as small a difference as possible (obviously),  but a few CM aren’t going to change anything appreciably to anything but an oscilloscope. We’re not quite approaching Planck constants for time or length, but what human auditory perception and spatial orientation will define.

this does seem to be a hobby for Asperger’s syndromish fussbudgets, so, you do you, enjoy. Keep us posted on your results.

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. 

For people working towards or that have an audiophile system they must be concerned with all aspects of a system that may have an impact. In the end every minute detail matters. But, as the OP pointed out this is not a highly resolving system. If you start working towards one, then, one of the steps will be to buy high end speaker cables… at that point they should be of the same length.

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.

Almost all speaker cables manufacturers sell them in equal lengths. Now if you are a diy hobbyists it doesn't natter.