My answer to this is similar to many other issues in audio - it depends. Speaker cables generally get more attention than they deserve. Compared to the bigger issues like room acoustics, the quality of recordings you listen to and the accuracy of your speakers, the differences between cables are relatively minor. The real question that most should ask is "are the differences worth my attention right now?" If you can answer all these questions with a definite yes, then continue on to focus on cables.
1. Is your room acoustically treated, including as many bass traps as you can fit and a sensibly chosen mix of diffusion and absorption? If not, address those issues first, the entire system will improve dramatically.
Take the free bass trap test
Confessions of a bass trap luddite
2. Do you have the best speakers you can afford?
3. Does your system impress you with the way that it can place musicians in the room, or even transport you to another venue? If not, you have other more important things to sort out first. A system should image well with even the most basic speaker cable.
4. Can your system reproduce bass with depth, power and a tight punch without any hint of boom? Don't accept missing or poor quality bass!
5. Can your system reproduce all the kinds of music you enjoy equally well?
6. Do you have albums that you enjoy both for their musical content and for the quality of the recording? If not, you're better off searching for new music.
Did you manage to get this far and answer yes in all cases? By now you've probably realised I view speaker cables upgrades as the icing on the cake for a system that is already well rounded and well put together. However what many audiophiles like to do is like the teenager who goes and spends all his money on the best tires and mag wheels that he can afford. Unfortunately his bomb car will never really gain much benefit!
Now the next question is what matters about speaker cable?