puzzle creation - Are there "rules" (or consistent conventions) that limit when the NY Times Crossword can deviate from rotational symmetry?

Most American crosswords use rotational (or "radial") symmetry so that squares directly opposite each other (through the center) are identical.

In the Times, I've only seen this not be the case when the layout was directly linked to the theme. Is that one of the "rules" you can count on, or are there (modern, Will Shortz era) examples of cases where you can't assume that non-radial symmetry will be somehow related to the theme?

Here's an example where the theme does justify the deviation:

The theme in this example was celebrating the 50th anniversary of Frank Lloyd Wright's landmark Guggenheim building.

Answer

Nearly all the Times crossword grids have rotational symmetry: they can be rotated 180 degrees and remain identical. Rarely, puzzles with only vertical or horizontal symmetry can be found; yet rarer are asymmetrical puzzles, usually when an unusual theme requires breaking the symmetry rule. This rule has been part of the puzzle since the beginning; when asked why, initial editor Margaret Farrar is said to have responded, "Because it is prettier."

SOURCE: "The Ins and Outs of Across and Down" The New York Times Magazine, 1992-02-16.