A B737-300 PIC RELEASES THE PARKING BRAKE AT THE GATE PRIOR TO WHEEL CHOCK INSTALLATION BY THE GND CREW AT ZZZ; US.

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PULLING INTO THE GATE WITH GATE SVCS PROVIDED; I NOTICED 2 RAMP AGENTS; ONE READY TO APPLY GND PWR; AND 1 MARSHALLER. AFTER STOPPING AT THE GATE; WITH #2 SHUT DOWN; LOOKING INSIDE TO SEE IF WE HAVE PWR; LOOKING OUTSIDE AT THE MARSHALLER; LOOKING INSIDE FOR PWR; LOOKING OUTSIDE TO MAKE SURE JETWAY IS MOVING; ETC; I END UP WITH BOTH ENGS SHUT DOWN AND GND PWR APPLIED. THE MARSHALLER GIVES ME A SIGN AND GETS DOWN OFF THE TUG. I RELEASED THE BRAKES; AND FEEL THE ACFT MOVE; SO I IMMEDIATELY REAPPLY THE BRAKES. I MAY HAVE FELT THE ACFT MOVE BTWN THE CHOCKS; BUT AFTER THINKING ABOUT IT FOR A WK; I THINK HE GAVE ME A HOLD BRAKES SIGN AND THEN LEFT MY SIGHT TO PUT THE CHOCKS IN. IT IS COMMON FOR 2 RAMP AGENTS TO CATCH AN AIRPLANE USING GATE SVCS. I DON'T KNOW IF THEY HAVE A SPECIFIC ORDER TO ACCOMPLISH THE MANY TASKS; BUT IN MOST PLACES IF THE CHOCKS WERE APPLIED FIRST; THE JETWAY WOULD BE STOPPED AND PWR APPLIED WITHOUT ANY ADDITIONAL DELAY. AS FOR MY OWN ACTIONS; THERE IS NO EXCUSE FOR RELEASING THE BRAKES WITHOUT THE APPROPRIATE SIGNAL; SO I LEAVE THE PARKING BRAKE SET UNTIL IT'S TIME TO RELEASE IT FOR PUSH. I KNOW IT WAS COMMON PRACTICE TO RELEASE THE PARKING BRAKE EVERY LEG AFTER THE CHOCKS WERE IN; SUPPOSEDLY FOR BRAKE COOLING; BUT IF THIS IS GOING AWAY WITH THE NEW CHKLIST; I THINK THIS WOULD BE A GOOD TIME TO CHANGE THE PROC; ESPECIALLY SINCE THE GATE SVCS BLOCK IN IS A BUSY EVOLUTION; STILL SEEN ONLY AS AN EXCEPTION. IF THERE IS A VALID REASON TO RELEASE THE BRAKES ON A THROUGH FLT COULD YOU LET US KNOW WHAT IT IS?