B757 CREW WAS DISPATCHED WITH AN INOP FUEL QUANTITY GAUGE.

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WE DEPARTED LAS VEGAS WITH AN ACFT THAT HAD AN MCO (MAINT CARRY OVER) ITEM RELATING TO THE L MAINT TANK FUEL QUANTITY INDICATION. IT WAS NOT A PROB THAT RENDERED THE L SYS INOP OR EVEN REQUIRED THAT DRIP STICK READINGS BE MADE ON THAT TANK. DURING THE PREFLT; WE NOTED THAT THE L AND R FUEL TANKS HAD BEEN FUELED PROPERLY AND THAT THEY WERE READING EQUALLY; ACCORDING TO THE GAUGES. SHORTLY AFTER LEVELOFF; THE FUEL CONFIGN LIGHT CAME ON AND WE NOTICED THAT THERE WAS A DIFFERENCE BTWN THE L AND R GAUGES. OUR INITIAL THOUGHT WAS THAT THIS WAS ERRONEOUS AND WAS A RESULT OF THE PREVIOUS MCO (MAINT CARRY OVER) ITEM. I THEN CLICKED OFF THE AUTOPLT AND NOTICED THAT THE L WING WAS HEAVIER THAN THE R WING. THE FO AND I THEN DECIDED TO XFEED FROM THE L TO R SIDE; ASSUMING THAT THE TANKS REALLY WERE OUT OF BAL. THE FO REACHED UP AND OPENED THE XFEED VALVE AND TURNED OFF THE R PUMPS. WE BEGAN TO CORRECT THE IMBALANCE; BUT THERE WAS STILL SOME DIFFERENCE UPON LNDG. WE WROTE UP THIS NEW PROB AND HAD MAINT AT LAX LOOK AT THE SIT. THEY COULD NOT FIND A REASON FOR THE FUEL IMBALANCE. MY CONCERN IS THAT THE ORIGINAL MEL ITEM DID NOT REQUIRE THAT THE TANKS BE CHKED WITH DRIP STICK READINGS. I THINK ANY TIME YOU HAVE A FUEL PROB OF ANY KIND THAT DRIP STICK READINGS SHOULD BE MADE AS A PRECAUTIONARY BACKUP. HAD DRIP STICK READINGS BEEN TAKEN AT LAS; IT MIGHT HAVE DETECTED THE PROB EARLIER.