As Sean wisely states, use what sounds best in your system.
Like Mattybumpkin, I use silver interconnects and copper speaker cable. I also use a silver power cord. Up until recently, I also used silver speaker cable, but found that I need copper somewhere in my system to keep the sound from crossing the line into an area I am no longer happy with.
While it is true that the ideal situation would be use the same wire from one end of a system to the other, it's much more complicated and difficult than most of us realize. Think about it, ever resistor, wire, coil, connector, jack, etc. would have to be the same. That would exclude all but a lunatic. Still, most of us have seen the benefits of maximizing the cabling of our systems, which is not so hard.