Is it code or recommended that the piping coming into and out of the mixing valve be copper and not CPVC?

Not as far as I know. If you have copper pipe it would be copper. If you have CPVC pipe, it would be CPVC.

Hi John:

There is no code requirement regarding this matter. As Harold stated, if you have brass valve then use copper pipe from valve and then transition to cpvc as needed. If have brass valve with cpvc fittings built into it then go cpvc all the way... ;)

Good luck...

MARK