That trimming is just done to make them easier to handle. By the end of winter the foliage of irises may be a bit ragged and in need of cutting back, but that will depend on the specific type of iris you are growing and how rough of a winter you have. I would not worry about trimming irises in the south until the end of winter when you can better assess their appearance. Just don't wait too long to cut them back as the foliage is needed to provide nourishment to support the spring (and in most cases also summer) blooming plants.
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