Is a full traffic pattern required when there is no traffic at an untowered airport?
See AC 90-66A - RECOMMENDED ‘STANDARD TRAFFIC  PATTERNS AND PRACTICES FOR AERONAUTICAL OPERATIONS AT AIRPORTS WITHOUT OPERATING CONTROL TOWERS  for information about uncontrolled airport traffic patterns.

It includes the following which says that while the FAA recommends using the full pattern, it is not required:


  7. GENERAL OPERATING PRACTICES.
  
  a. Use of standard traffic patterns for all ‘aircraft  and CTAF procedures by radio-equipped aircraft  are
  recommended at all airports without operating  control towers.
  However, it is recognized that other  traffic patterns may already be
  in common use  at some airports or that special circumstances or 
  conditions exist that may prevent use of the standard  traffic pattern  


And then:


  e. The FAA encourages pilots to use the standard  traffic pattern.
  However, for those pilots who choose  to execute a straight-in
  approach, maneuvering for  and execution of the approach should be
  completed  so as not to disrupt the flow of arriving and  departing
  traffic. Therefore, pilots operating in the  traffic pattern should be
  alert at all times to  aircraft executing straight-in approaches.  
  
  f. Pilots who wish to conduct instrument  approaches should be
  particularly alert for other aircraft in the pattern SO as to avoid
  interrupting  the flow of traffic. Position reports on the CTAF 
  should include distance and direction from the  airport, as well as
  the pilot’s intentions upon completion of the approach