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.