COMMUTER LTT ASSIGNED 350 DEG HDG TO V25. CAPT; PF; NOTED THEY WERE ON THE 141 DEG RADIAL OSI; V25; AND BEGAN TRACKING INBOUND. CTLR WANTED ACFT TO REMAIN ON THE 350 DEG HDG AND THE FLC COMPLIED. FATIGUE FROM A DELAYED DEP DUE WX SFO AND FAMILIARITY WITH NORMAL ROUTING.

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WE DEPARTED MRY TO SFO ON AN IFR FLT PLAN; BUT WERE IN VFR CONDITIONS SHORTLY AFTER TKOF. ATC WAS VERY BUSY WITH ACFT AS A GND STOP AT SFO HAD JUST BEEN REMOVED. WE WERE THE FOURTH SCHEDULED FLT OUT OF MRY IN 20 MINS. AFTER CONTACT WITH DEP CTL HE ASSIGNED US A HDG OF 350 DEGS TO INTERCEPT V25 (TO OSI). THE USUAL ROUTING IS A HDG TO INTERCEPT THE OSI 141 DEG RADIAL WHICH IS A PART; BUT NOT ALL; OF V25. THERE IS A DOGLEG ON THE AIRWAY THAT GOES TO SNS. I HAD THE OSI 321 DEG RADIAL IN MY OBS AND NOTED THAT I WAS ALMOST CTR ON IT; SO I BEGAN TO TRACK THE RADIAL INBOUND. SHORTLY AFTER; THE CTLR QUESTIONED MY HDG. WHEN I TOLD HIM THAT I WAS TRACKING THE 321 DEG RADIAL; HE RESPONDED THAT THAT WAS NOT THE CLRNC. I TURNED TO THE ASSIGNED HDG AND PROCEEDED AS CLRED. THERE WAS NO SEPARATION PROB APPARENT. THIS INCIDENT OCCURRED BECAUSE OF A FEW 'SMALL' FACTORS: 1) LAST LEG OF A 2-DAY (AND NIGHT) TRIP -- GET HOMEITIS AND FATIGUE. 2) FO BUSY TALKING TO THE COMPANY. 3) THE CLRNC WAS VERY SIMILAR TO THAT USUALLY GIVEN. 4) THE EXPECTATION OF THE 'USUAL.' WHEN FLYING COMMUTERS TO THE SAME ARPTS AT THE SAME TIME ALMOST DAILY; ATC INSTRUCTIONS CHANGE VERY LITTLE AND IT IS EASY TO BECOME COMPLACENT. PLTS FLYING SUCH TRIPS NEED TO BE REMINDED TO GUARD AGAINST IT WHICH COULD HAVE PREVENTED THE ABOVE INCIDENT.