Does the length of cables really matter?

Yes, I've heard that to about digital cables. But then it's strange that the manufacturers make them shorter. Or...?

As I recall, there's a really short length, like half a foot, where the reflections don't matter.  I don't remember where I read this though.  Maybe on Wireworld's site?

Coaxial digital cables do have ideal lengths to minimize reflection and jitter. Those reflections are caused by the receiver impedance changing radically and causing reflections. Analog inputs do not have this sort of behavior, the impedance is constant.

I’ve never heard of a minimum requirement for power cables, but I do think having clean power, keeping your wall-warts out of your clean zone, and using shielded power cables can ensure the best performance.