When may commercial pilots release their shoulder harness?
You essentially have two questions, which I will answer in turn:


When may commerical pilots release their shoulder harnesses when operating under commercial operations rules such as §121
When do such pilots actually release their harnesses in practice




Regarding your first question:

14 CFR 91.105 does generally apply to commercial operators and air carriers. However, §121, §135, and §137 each provide equivalent regulations which duplicate or exceed the language found in §91.105.

14 CFR 91.105 states, in pertinent part:


  (b) Each required flight crewmember of a U.S.-registered civil
  aircraft shall, during takeoff and landing, keep his or her shoulder
  harness fastened while at his or her assigned duty station. This
  paragraph does not apply if—
  
  (1) The seat at the crewmember's station is not equipped with a
  shoulder harness; or
  
  (2) The crewmember would be unable to perform required duties with the
  shoulder harness fastened.


14 CFR 121.311 states, in pertinent part:


  (h) Each occupant of a seat equipped with a shoulder harness or with a combined safety belt and shoulder harness must have the shoulder harness or combined safety belt and shoulder harness properly secured about that occupant during takeoff and landing, except that a shoulder harness that is not combined with a safety belt may be unfastened if the occupant cannot perform the required duties with the shoulder harness fastened.


14 CFR 135.171 states, in pertinent part:


  (b) Each flight crewmember occupying a station equipped with a shoulder harness must fasten the shoulder harness during takeoff and landing, except that the shoulder harness may be unfastened if the crewmember cannot perform the required duties with the shoulder harness fastened.


14 CFR 137.42 states, in pertinent part:


  No person may operate an aircraft in operations required to be conducted under part 137 without a safety belt and shoulder harness properly secured about that person except that the shoulder harness need not be fastened if that person would be unable to perform required duties with the shoulder harness fastened.


Under each regulation, the flight crewmember may generally unfasten his or her shoulder harness at any times other than during takeoff and landing, or if the shoulder harness would interfere with performing required duties. The one exception to these allowances is found under §121 in instances where the shoulder harness and safety belt are combined1.

These allowances notwithstanding, a certificate holder may include in its General Operating Manual provisions that flight crewmembers must adhere to requirements that exceed those given by regulation.



Regarding your second question:

In practice I never release my shoulder harnesses from the time I seat myself and fasten my safety belt and shoulder harness until the aircraft has come to a stop and I am ready to leave my seat at the end of the flight. I have also never seen a fellow pilot at my air carrier release his or her shoulder harnesses while in flight. Others will doubtless have different experiences.

Prior to my time with my present employer I have witnessed pilots who make a practice of not wearing the installed shoulder harness at all (given the circumstances I don't think they would have had a very good argument that it would otherwise interfere with performing their duties). I have also witnessed pilots who unfasten both the shoulder harness and safety belt after landing while still taxing at the end of a flight. This later practice is not against regulation2 but may be of questionable judgment in some cases.



1 §121 requirements for combined safety belt and shoulder harness:

14 CFR 121.311 also gives the following, in regards to the requirements for aircraft to be equipped with combined safety belts and shoulder harnesses:


  (f) No person may operate a transport category airplane that was type certificated after January 1, 1958, or a nontransport category airplane manufactured after March 20, 1997, unless it is equipped at each flight deck station with a combined safety belt and shoulder harness that meets the applicable requirements specified in §25.785 of this chapter, effective March 6, 1980, except that—
  
  (1) Shoulder harnesses and combined safety belt and shoulder harnesses that were approved and installed before March 6, 1980, may continue to be used; and
  
  (2) Safety belt and shoulder harness restraint systems may be designed to the inertia load factors established under the certification basis of the airplane.


I read this to mean that, for aircraft specified under §121.311(f) above, unless a shoulder harness was approved and installed before 6 March 1980, then the aircraft must be equipped at each flight deck station with a combined safety belt and shoulder harness. Furthermore, under the requirements of §121.311(h), when the flight deck stations is so equipped with the combined safety belt and shoulder harness, no provision of §121 allows for that shoulder harness to be unfastened during takeoff and landing, even if the shoulder harness would interfere with performing required duties. Presumably the approval process for those harness systems would ensure that no interference would be possible.



2 14 CFR 91.107 does generally require that each person on board a U.S.-registered civil aircraft must have a safety belt and any installed shoulder harness properly secured about him or her during movement on the surface. However, this requirement does not apply to required flight crewmembers or to any operations conducted under §121, §125, or §135. See §91.107(b).