I have read the different opinions on this subject, and now for my experience (or 2 cents).
The cables you get in the box work, are they good? Not really. First, if you are doing any kind of system that has to be put into a rack or cabinet, chances are they may not be the right size. Second, it is rare to find shielded cables that come with equipment. With all the other cables and power cords going behind the equipment, you should have shielded cables - your systems may not need them all the time but it doesn't hurt to be cautious. Third, how many trips back for sevice do you want to make? The cheapy cables typically have cheap ends and are not reliable connections, the client moves the DVD to clean under it and the audio connections fall off. He isn't going to want to pay you for a sevice call to re connect cables that in his mind shouldn't have fallen off.
Next, the debate about how much of a difference better cables make for sound and picture.
The picture side is easy, you can see the difference.
Cheap cables can cause the picture to look grainy, a halo effect and many other issues. This is something easy to test in your showrooms or at home. Even S-video cables are not all the same. The better the display the easier it is to pick out the difference. Keep in mind the picture is only as good as the source. There are lots of places to find out about good test material, even this site should have a thread somewhere about this.
The audio side is not so easy. In most basic systems good interconnects "can" help. Things to listen for would be better base response and the "sound stage" should open up a little more. Not all systems will sound different, you may aready be getting all that it can do. As you move up to higher end gear and high end speakers the effects can actually be dramatic. Anyone that has had to chance to play with equipment like Wilson/JM Labs/MBL/Martin Logan/Mark Levinson/Boulder/Theta and the many other exotic brands will know first hand about the wow factor of using expensive cables and speaker wire.
Keep in mind, and most that know will agree, it's all relative to the capabilities of the equipment.
It's also a sale, if you are trying to offer your client a system that is easy to use and reliable then cables that are good and the right length (or custom made)add to the value of his system and your bottm line.