On 4 Jun, 22:10, "George" wrote:

If all sides are equal then squareness is no more than using a set square on
each corner.

Overall for the finished product, I care less about corner accuracy
and more about overall symmetry.

It's true that "Four 90° make a rectangle", but only true in error-
free geometry. Out in the real world we have to account for errors
too, and it's simple arithmetic / error analysis to show that
aggregating three inaccurate corners is likely to end up with very
poor overall accuracy. Measuring the diagonals though is easier (most
of us have better tape measures than goniometers) and it's a more
direct reading of the accuracy factor we most care about. As a result,
it gives better results when framing.