Right, I see where much reference to square rule assumes a reference face. My take on it is that if I am laying out the table of a housing to be parallel to ANY reference plane, be it a face, offset line or centerline, I would consider it square rule. That's just my personal view and could be a modern interpretation; I appreciate the historical and cultural precedents you and Jay mention. I've used centerlines when the designer has dimensioned to them, and fully housed the joinery parallel to them.
We have a big timber framing course starting tomorrow so may decline to post further for awhile, but I'm glad we have a good sawyer that gives us timber that is within our 1/8" tolerances (I agree with that limit) so we can use edge layout. Using white pine also helps; our last batch of red pine twisted like crazy; never again.