How much difference in heading/bearing between consecutive legs is "officially" considered a turn?
I will have to answer my own question, since I have noticed that in FAA's PBN Manual it states:


  PBN 8260.58A 1-2-5c . Turn parameters. For OEA construction, a turn is indicated when the course change exceeds the alignment tolerance of 0.03 degrees.


So, at least as far as route design is concerned, the alignment threshold for turn indication is 0.03 degrees.