My understanding is that the FAA ruled (or maybe it was the NTSB on somebody's appeal), if you are told to "fly heading 180, maintain VFR", you're still required to select an altitude that will keep you clear of Bravo. On the other hand, if you hear "flying heading 180, maintain 5000", the law would consider that an implied clearance.
But if you're like me and prefer not to get the lawyers and judges involved, get the "magic words" on the tape, or file IFR.