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

The answer to this question is maybe. You do not want too much impedance mismatch between the left and right cables, because that can cause a difference in the frequency response of each of the speakers.  So if the cables are both relatively short and the distributed impedance is minimal then the lengths can vary significantly without ill effects.

Is there any scientific reason that different speaker cable lengths will make a detectable difference in sound quality - no.

Will you be able to notice any sound quality difference with different cable lengths - no.

As an audiophile will you always wonder if you are not getting optimum sound quality partially because the speaker cables are not the same length - absolutely.

Therefore get them as short as possible and equal lengths (and like someone else posted - it helps with resale value).

I have always kept both speaker cables the same length assuming that they had to be for reasons of timing i.e. the time it takes the signal to travel from the amp to the speaker or maybe for other electronic reasons? However some cables are extremely expensive so buying cable that is not needed is costly. So if your amp is not placed in the middle i.e. equidistant from each speaker (mine is next to the right hand speaker) you could get away with having a much shorter cable one side than the other. In my case I could have a 1 metre cable from the amp to the right speaker and a 3.5 metre cable going to the left speaker but I have never tried it. I was considering buying Townshend Fractal 1 cables to replace my Townshend Isolda cables but didn’t get them in the end due to finances but I was assured by the distributors for the cables that there was no problem whatsoever in using cables of unequal length. I am not sure if this is just because of the type of cables these are or if it is true of all speaker cables though. As for the timing issue I thought about well obviously unless there is a massive difference in length (far greater than would be used in a domestic hi-fi setting) you would never hear the difference in the time the signal arrives at each speaker as it is almost instantaneous on both channels irrespective of whether the cable is 1 metre long or 5 metres long so to your ears both speakers produce the sound at exactly the same time. Anyone who believes that their hearing can detect the difference in timing that is measured in a microsecond or two is quite plainly wrong, they cannot.

If, like the OP, your amp is much closer to one speaker, and you elect to have equal length cables, then what is the best way to handle the excess cable going to the speaker nearest the amp?

And they sb really expensive.