Can a pilot initiate an enroute descent to an IAF without ATC 's approval?
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.