I think maybe my eyes need adjusting as to me, this image is not in focus. There are a few parts of the leaves toward the middle that are sharp and crisp as is the stem where you appeared to have focused, but the flowers, not so much at all. I am going to surmise you used a 1.2 or 1.4 lens to shoot this and in focusing on the stem part of the flower, you killed your outside flower petals focus.
As to the rule of thirds...to quote Pirates of the Caribbean, "aye, we merely see them as guidelines." There are any number of formats on can use to determine the placement of an object in a crop. Personally, I lend toward the Golden Spiral and Trinagles, both of which can be found in CS6. On your image, I used Triangles because I thought it best centered the object as a positive object in a negative space in such a manner as to have the diagonal of the stem work favorably with the linear pushing out of the leaves and flower petals. As always, it is a matter of personal opinion and taste.
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And it probably could have stood a bit more off the right side.