Mixing Silver and Copper cables -- Good or bad??

I have heard from a highly respected retailer that mixing silver interconnects with copper speaker cables is bad -- that you can actually hear the problems at the interface where it switches from silver to copper or vice versa. (what about the copper and/or silver wiring in the amplifier, wouldn't that have an effect as well?) he said one should use all copper or all silver throughtout the system. Is this right? Or is there a balalnce to be had? If a silver IC is noticably superior to a copper one, would that advantage be worth more than the problem casued by the silver IC to copper speaker cable interface in the system?

Finally, if you are bi-wiring or bi-amping a set of speakers, I have heard it could make sense to run silver on the top, and heavier copper on the bottom. Is this correct? Or, as the retailer would suggest, that mixing silver wires with copper wires is the overwhelming no-no?

What has been people's experience mixing silver IC's with copper speaker cables (or vice versa!)? What about silver on the high and copper on the low in bi-wire or bi-amp set-ups? What does theory, practice and experimentation tell us about this?