Okay here we go..............in serious mode.
Although I am reasonably new to home cinema, I have run my own electronics company for many years and believe me good quality cables and connectors are important.
When choosing a proprietary interconnect cable, it is important to buy from a source that uses good quality cable correctly terminated to good quality connectors. If you are handy with a soldering iron, or have the correct crimp tools, there is no reason why you should not make your own interconnects using good quality professional cable, from a reputable manufacturer such as Van Damme as mentioned on another current thread, which is designed for the application. There are cables specifically designed for video, analogue audio, digital audio, speaker etc. Mate these cables with good quality professional grade connectors and you will have top quality interconnects as used by professional studios, BBC et al.
With regard to cable length, I agree with Bob007 that the best length of cable is one that reaches between the two components to be joined. If interconnects are longer than necessary, there is more risk of picking up any interference that may be around. With speaker cables I agree with Juboy that IN THEORY all cables should be the same length i.e. the same resistance but in a domestic situation it is irrelevant. The difference in resistance of various speaker cable lengths in the average room is a bit like standing on your roof to get a better look at the moon.
To summarise, yes, good quality cables are important but you dont need to spend a fortune to get them.
This is my personal opinion, other may disagree.