B737 FLT CREW REFUSES ATC INSTRUCTION TO DSND TO LOWER CRUISE ALT DUE TO FUEL CONCERNS.

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I WAS THE PLT MONITORING. WE WERE CRUISING ENRTE TO PHX INITIALLY AT FL410. ZAB HAD US DSND TO FL380; THEN FL340. CTLR TOLD US THE RIDE SHOULD BE BETTER AT FL340 (IT WASN'T -- LIGHT CHOP). CTLR DIRECTED DSCNT TO FL320. CAPT JUMPED ON THE RADIO AT THIS POINT AND TOLD CTLR WE WERE STAYING AT FL340. CTLR QUESTIONED THE RESPONSE. CAPT REPLIED THAT WE WOULD STAY AT FL340 AND THAT WE DID NOT WANT FL320 FOR A BETTER RIDE. THIS EXCHANGE WAS REPEATED AND CTLR ASKED WHETHER THIS FAILURE TO COMPLY WITH THE CTLR INSTRUCTION REQUIRED SUPERVISORY ATTN/PHONE NUMBER. WE DSNDED TO FL320 AND WERE SWITCHED TO NEW CTLR. NEXT CTLR PASSED US WATCH SUPVR'S PHONE NUMBER TO CALL WHEN WE GOT ON THE GND. I RESUMED RADIO DUTIES AT THIS POINT. WE HAD TAKEN OFF APPROX 5000 LBS HVY AND CAPT HAD COORD WITH DISPATCH PRIOR TO DEP. DESPITE CRUISING AT A HIGHER ENRTE ALT THAN FILED FOR FUEL CONSERVATION THE FMC WAS SHOWING ARR WITH RIGHT AT 5000 LBS FUEL REMAINING. I BELIEVE THE CAPT'S CONCERN FOR OUR FUEL STATE RESULTED IN HIS DESIRE TO REMAIN AS HIGH AS POSSIBLE AND THAT THIS CONCERN WAS NOT UNDERSTOOD BY THE CTLR. ENSURE OPERATIONAL CONCERNS THAT AFFECT DESIRED HANDLING BY ATC ARE CLRLY COMMUNICATED AND UNDERSTOOD.