The comment earlier on about the variation in speaker cable allowing for hiding under carpets (flat), in walls (thicker sheathing), is true.
The argument for spending money on 'top quality' cable is the same argument for spending �1000 on a CD player.It's somewhat unneccesary to enjoy most of your cd collection, and the law of diminishing returns applies.
For most of us, who have 'normal' hi-fi's, I suppose bell wire is okay (I don't advise mains cable, simply because it's too thick and doesn't bend where you need it to. ) I've used mains cable in a monitor system in a recording studio. No harm done.
If you're the sort of person who spends �4000 on a hi-fi system, you'll probably want to have a listen to it first. There are a number of factors which define the sound of a system. In decending order, these are likely to be ;
1)Speakers
2)The room in which you listen
3)The quality of the music you play
4)The amplifier / cd
5)Whether you are awake or half asleep
99) The speaker cable.
This said, I have been in listening rooms and listened to the difference between a bog standard cable, and a top quality cable on an expensive system. And the difference is small. I say small, I mean that the trained ear can just about recognise the difference.
Question in that scenario is - Is that small difference worth the �250 extra that the salesman is trying to push?