Can a pilot initiate an enroute descent to an IAF without ATC 's approval? An IFR flight is cleared DIRECT to a published IAF for an approach at the destination airport. The cruising altitude is 10,000’, the IAF crossing altitude is 2,000’ and the T/D begins about 24nm prior to the IAF with standard rate of descent.

Since route has been previously cleared to that IAF waypoint, can the pilot commence his descent upon reaching the T/D, or wait to hear from controller or request descent instructions just prior to the T/D?
You are never allowed to simply vacate an assigned IFR altitude without clearance.  When you were cleared to the IAF your clearance should have included "pilot's discretion down to ..." of "cross IAF at or above..." or similar words.  If you don't get this, just ask.  Never presume that you can just start down on your own though.