sacul12,
I agree that it's a good idea to switch to TRUE on the ground, to check that the ITT in the FMS matches the OFP.
In the case of a change to the flight plan after take-off, you would have to enter the new waypoints into the FMS and cross-check the ITT and distance against a reference publication, such as a table of tracks and distances.
These new waypoints, ITTs and distances would then be written down, perhaps on the OFP, 'overwriting' the original plan.
Setting the HDG REF to TRUE enables an extra check that the above process has been correctly accomplished. As each waypoint is crossed, the actual track and distance to the next waypoint can be checked against the revised plan.
As the OTS Track Message says, "85% of gross navigational errors occur after a change in routing" (or words to that effect).
To keep the SOP simple, it may be a good idea to always select TRUE when navigating in NAT airspace, even if not required by regulations.
Just my 2-cents' worth.