CRJ200 Captain on the ILS PRM behind a B757 reports slowing prior to the marker after being instructed by ATC to maintain 180 to the marker.

Narrative

On an ILS PRM; we were told to hold 180 KTS to the marker; caution wake turbulence for a B757 in front of us. The 757 was 5 miles in front on TCAS. As the 757 hit the marker he slowed rapidly and quickly showed inside 5 on the TCAS then inside 4 on the TCAS with the blue diamond turning a solid blue. He was still slowing and approaching approximately our 3 1/2 mile range. At this point we were 2 miles from the marker and I became concerned from a safety aspect of encountering wake turbulence while in IMC and or receiving an RA from the TCAS forcing a go around. I made a conscious decision to begin slowing and prevent a safety incident I felt was about to occur. Landing was uneventful; but Ground Control had us call TRACON were I talked to a Supervisor concerning our early slowing when assigned a speed. I completely understand the ILS PRM procedure and the requirements; but on this occasion felt that the situation had deteriorated to a point of being unsafe for our aircraft. The Controller said we had been 6 out from the 757 but when we made the decision to slow the 757 was well within our 5 mile ring and our symbol had turned solid blue and was still getting closer. The Supervisor did understand my concern; but reminded me to comply with the assigned speeds. I completely concur with his concerns.