MD80 ON APCH TO ATL SLOWS BELOW ASSIGNED SPD.

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EVENT OCCURRED ON DEC/THU/04 ABOUT XA00. AFTER JOINING THE RWY 9R LOC FOR APCH TO RWY 9R AT ATL; OUR SPACING WITH THE ACFT AHEAD SEEMED VERY CLOSE -- LESS THAN 3 MI. APCH CTL ASSIGNED A SPD OF 180 KTS TO THE OM. FLT CONDITIONS WERE BELOW THE CLOUDS IN MODERATE RAIN AND AT NIGHT. WIPERS WERE NEEDED TO MAINTAIN ADEQUATE VISIBILITY; WITH THE ASSOCIATED NOISE LEVEL BEING A HUGE DISTR. BECAUSE OF MY CONCERNS ABOUT BEING TOO CLOSE TO OUR TFC AHEAD; AT ABOUT 4 MI FROM THE MARKER I SLOWED TO 160 KTS. AT ABOUT 2 MI FROM THE MARKER; APCH CTL ASKED US WHAT OUR SPD WAS AND I TOLD HIM 169 KTS; TO WHICH HE REPLIED HE HAD ASSIGNED 180 KTS TO THE MARKER. WE ACCELERATED BACK TO 180 KTS TO THE MARKER. THE ACFT AHEAD CLRED THE RWY AS WE PASSED 500 FT. ON SHORT FINAL; WE ENCOUNTERED SIGNIFICANT WAKE TURB (TFC WAS AN MD80) APCHING THE POINT OF DESTABILIZING THE APCH. MY REASON FOR SLOWING BELOW THE ASSIGNED SPD WAS MY CONCERN FOR BEING TOO CLOSE TO THE ACFT AHEAD. AGAIN; FROM TCASII AND VISUALLY IT APPEARED LESS THAN 3 MI. TFC WAS HVY AND I ASSUME APCH WAS LINING TFC UP WITH MINIMUM SPACING. IN THESE SITS; I HAVE IN THE PAST BEEN 'CTLED' INTO A GAR BECAUSE THE SPACING GOT TOO CLOSE AND THE ACFT AHEAD WAS NOT ABLE TO CLR THE RWY BEFORE MY LNDG. I WAS TRYING TO AVOID THIS FROM HAPPENING.