COMMUTER FLC MISUNDERSTANDS ALT ASSIGNMENT DEPARTING PHL.

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WE DEPARTED PHL ON THE PHILADELPHIA 6 DEP WHICH SPECIFIES 5000 FT IF NO LOWER ALT IS ASSIGNED. AFTER XFERRING FROM DEP CTL TO THE NEXT ENRTE CTLR; AND THEN LATER AFTER LEVELING AT 5000 FT; ATC QUERIED IF THE PREVIOUS CTLR HAD ASSIGNED US 5000 FT BECAUSE HE SHOWED WE SHOULD BE AT 4000 FT. WE REPLIED THAT 5000 FT WAS THE ASSIGNMENT. HE SAID THAT THERE WAS NO PROB; AND TO DSND TO 4000 FT 'AT PLT'S DISCRETION.' THE CONCERN IS THAT AFTER SWITCHING FROM TWR TO DEP; AND THEN DEP TO THIS CTLR WE RPTED 'THROUGH X ALT FOR 5000 FT.' SO THEREFORE THERE WERE 2 OCCASIONS WHERE THE MISUNDERSTANDING COULD HAVE BEEN CAUGHT. THIS IS WORRISOME BECAUSE OF THE FAA'S POSSIBLE NEW POLICY HOLDING PLTS IN ERROR WHEN THEY REPEAT CERTAIN DIRECTIVES BACK AND ARE NOT CORRECTED BY ATC THOUGH THE READBACK WAS INCORRECT. THIS ELIMINATES A CHK AND BAL THAT IS VITAL TO THE ATC SYS. I BELIEVE IN THE LONG RUN IT WILL PROMOTE COMPLACENCY IN CTLRS AND MISTAKES WILL NOT BE CAUGHT AS IN THE EXAMPLE ABOVE. MY VOTE IS TO RETAIN RESPONSIBILITY ON BOTH SIDES WITH A RENEWED SENSE OF COOPERATION TO PROMOTE SAFETY.