B767-300ER CREW; DEPARTING SEA; TURNED PREMATURELY MISSING A DME XING RESTR.

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DEP OFF RWY 16L: 'MOUNTAIN 5 DEP PROC.' 'INTERCEPT AND PROCEED VIA SEA 161 DEG RADIAL; CROSS D5 SEA MINIMUM 3000 FT; THEN TURN TO 070 DEG HDG FOR RADAR VECTORS TO ASSIGNED RTE.' CAPT (PF) WAS ON RAW DATA TO ENSURE PROPER CORRIDOR TRACK FOR THE NOISE ABATEMENT PROC. I (PNF) WAS ON FMS DATA. AT 3000 FT MSL; THE CAPT STATED 'THERE'S 3' AND INITIATED A TURN TO 070 DEGS. AT THAT POINT; I WAS BUSY WITH CLB/CLEAN-UP DUTIES; TFC SPOTTING AND ATC COMS. IN THE TURN; I NOTICED THE FMS SHOWED AN EARLY TURN. I RECHKED THE DEP PROC; XCHKED THE CAPT'S HSI TO SEE WHAT IT WAS SHOWING AND WAS JUST ABOUT TO BRING IT TO THE CAPT'S ATTN WHEN ATC ADVISED US TO REVIEW THE PROC. THIS OCCURRED IN A MATTER OF SECONDS. AFTER DISCUSSION; WE DECIDED A BETTER COURSE OF ACTION WOULD HAVE BEEN FOR THE PF TO BE ON FMS AND THE PNF BACK IT UP WITH RAW DATA. THIS WOULD ALLOW FOR THE PF TO NOTICE AN EARLY TURN; OR THE NEW QUICK ALIGNMENT PROC FOR RWY 16L WOULD VERIFY FMS ACCURACY AND ALLOW BOTH PLTS TO USE FMS DATA. SUPPLEMENTAL INFO FROM ACN 533611: I STARTED THE TURN TO 070 DEGS AFTER CLBING THROUGH 3000 FT FORGETTING TO WAIT FOR THE DME RESTR.