A PIPER WARRIOR PLT NEARLY ENTERED SFO CLASS B AIRSPACE WITHOUT A CLRNC.

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I FLEW FROM CCR TO HAF VIA RICHMOND; OAK; THE SAN MATEO BRIDGE; AND SQL. BAY APCH INSTRUCTED ME TO PROCEED FROM RICHMOND AND TO CROSS THE MORMON TEMPLE AT 2500 FT MSL. AFTER PASSING THE MORMON TEMPLE; I WAS DIRECTED TO FLY TO THE OAKLAND COLISEUM AND TO CONTACT THE N OAKLAND TWR ON 118.3. I SWITCHED FREQS AND GAVE THE TWR MY FULL CALL SIGN AND ALT (STILL 2500 FT MSL). THE N TWR DIRECTED ME TOWARD THE 'NUMBERS' OF RWY 29 AND HANDED ME OFF TO THE S TWR ON 127.2. AGAIN I CHKED IN WITH MY FULL CALL SIGN AND INDICATED ALT (STILL 2500 FT MSL). THE S TWR CTLR INSTRUCTED ME TO FLY OVER THE 'NUMBERS' OF RWY 29 AND THEN TO THE MID-SPAN OF THE SAN MATEO BRIDGE. AT THIS POINT; I WAS CONCERNED ABOUT MY ALT (I WAS ABOUT TO ENCOUNTER A PORTION OF THE SFO CLASS B AIRSPACE WITH A FLOOR OF 2100 FT MSL); SO I ASKED THE CTLR TO '...VERIFY THAT ACFT X WAS CLRED TO ENTER CLASS B AIRSPACE AT 2500 FT.' HE RESPONDED NEGATIVELY AND SAID TO CROSS THE 'NUMBERS' OF RWY 29 AT 2000 FT -- THIS WAS THE FIRST ALT ASSIGNMENT I RECEIVED SINCE MY FIRST ONE FROM BAY APCH. I DSNDED IMMEDIATELY AND DO NOT THINK I PENETRATED THE CLASS B AIRSPACE; BUT IT WAS CERTAINLY CLOSE. I WOULD ENCOURAGE ATC TO BE CONSISTENT WHEN CTLING TFC WITHIN THE TIGHT QUARTERS OF CLASS B AND CLASS C AIRSPACE. FROM THE POINT OF VIEW OF THE PLT; WE MAINTAIN AN ATC-ASSIGNED ALT UNTIL 1) GIVEN ANOTHER ALT TO HOLD; OR 2) ADVISED TO CHANGE ALT AT OUR DISCRETION TO REMAIN CLR OF CLASS B AIRSPACE. SIMILARLY; I WOULD ALSO ENCOURAGE PLTS TO NOT BE LULLED INTO A FALSE SENSE OF SECURITY -- JUST BECAUSE WE'RE TALKING TO ATC DOESN'T MEAN THAT WE NO LONGER HAVE TO AVOID AIRSPACE WE DON'T BELONG IN.