When flying to a towered airport when the tower is closed, what frequency should I use?
You use the CTAF for announcing your position and intentions, i.e. 125.85. From the AIM 4-1-9(g):


  If an airport has a tower and it is temporarily closed, or operated on
  a part-time basis and there is no FSS on the airport or the FSS is
  closed, use the CTAF to self-announce your position or intentions.


4-1-9(b) says:


  The CTAF may be a UNICOM, MULTICOM, FSS, or tower frequency and is
  identified in appropriate aeronautical publications.


125.85 is marked as the CTAF on the sectional and listed in the Chart Supplement. 

Note: The letter "C" in the dark circle next to the control tower ("CT") frequency indicates that the CT frequency is the CTAF frequency at that airport.