It's been my observation that, when posted images (on forums) are critiqued, the same elements of the images are routinely discussed. These elements usually fall into three categories: Technical (e.g., exposure, focus, color, lighting), creativity and artistic values (composition, art direction, effects), the model (posing, makeup, wardrobe, she's hot or she's not.)
When discussing composition, the remarks, most often, have to do with the effectiveness of the framing or the crop, the angle or vantage point from which the image was captured (i.e., from the shooter to the model), and the Rule of Thirds. Rarely, do I see comments focused on symmetry. Yet, creative uses of symmetry are key elements that often cause a select, few images to stand apart from the many. I'm not saying many photographers aren't aware of symmetry in their images. I'm saying it doesn't seem to warrant much in the way of critical discussion.
In a few genres, like Fine Art Nude, symmetry can sometimes be THE key element to a successful shot! What often sets models' poses apart, from "okay" to "fantastic," is the way in which she creates symmetry with the curves and shapes of her body. It's up to the shooter to spot the symmetry in his or her model's poses, i.e., where the symmetry is beautiful and captivating, and capture those moments.
Here's how the dictionary definitions of "symmetry" read, at least those definitions that I would apply to pretty girl shooting:
sym·me·try (noun, plural -tries)
1. The correspondence in size, form, and arrangement of parts on opposite sides of a plane, line, or point; regularity of form or arrangement in terms of like, reciprocal, or corresponding parts.
2. The proper or due proportion of the parts of a body or whole to one another with regard to size and form; excellence of proportion.
3. Beauty based on or characterized by such excellence of proportion.
If I were writing a book on this pretty girl shooting thing, I'd probably devote an entire chapter to the subject of symmetry. What's more beautiful than the symmetrical curves of a woman's body? Nothing comes to my mind except maybe more digits before the decimal point in my checking account's balance. In fact, it's symmetry and balance that is most-perceived as the defining factor for human beauty, especially in the face.
In writing that book, I'd not only discuss the creative uses of compositional symmetry, I'd get into the use of asymmetical devices wherein line, shape, and form, as elements of an image's composition, are juxtaposed or work, dramatically, against each other to grab the viewer's attention.
Anyway, some food for thought.
The pretty girl shots of a JimmyD certified, "Pretty Girl," are of Coty. These were snapped about two years ago in the studio with a Canon 20D w/28-135 zoom.