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Pasar Kedai Payang, originally uploaded by Fadzly Mubin.
Taken with a Nikon D50 and Nikon 70-300mm f/4-5.6G lens

I don't think most photographers are aware of the rules of composition as developed during the Florentine Renaissance or the Beaux Art era (architects may be more familiar with them). The issues of horizon line, "thirds -- the grid, is the kindergarten version.

Unless the photographer is working in studio conditions where he or she has control over every aspect of and in the subject-frame like an art director, the photographer is limited to what presents itself to the camera and how.

In both instances the tendency will be to adhere to the rules, even if they are consciously unknown, due to cultural "osmosis".

Breaking the rules is so common that they have become rules themselves. This is most often seen, professionally, in advertising, book and magazine cover photography for example because it draws attention, tending to stand out. If the goal is to draw attention to itself, the rule is to break a (cultural and unconsciously expected) rule.

In varieties of candid photography, photography of the moment, there is little opportunity to compose according to the rules (that may be done later by cropping if possible). In such photography, the frame and subject 'emit' their own rule, which is what captured the photographer's attention and caused her to raise the camera and release the shutter in the first place. Since my photography is mostly of that sort, I tend to let the frame/subject compose itself.