Question: 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:

![enter image description here](https://i.stack.imgur.com/CQ2Hy.jpg)

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.](http://www.nytimes.com/1992/02/16/magazine/crosswords-fiftyyears.html?ei=5070&en=eb226eeaee1ab061&ex=1237089600&pagewanted=all)
Quality crossword puzzles have had a [rotational symmetry requirement](http://www.cruciverb.com/index.php?action=ezportal;sa=page;p=21) for a very long time. It was determined early on in the development of the puzzle that the symmetry rule (along with the fully checked and all-over interlock rules) made the puzzles more interesting. (IMHO, that is because it puts interesting constraints on the beginnings of words towards the upper left corner, with mirrors of those constraints on the endings of words in the lower right.)

The NYT does expressly give the editor freedom to break the rules, ["if the theme warrants"](http://www.cruciverb.com/index.php?action=ezportal;sa=page;p=16). But notably none of my searches for submissions guidelines to the NYT made any more explicit statements than that.