Cutting back the foliage of bearded iris is something that is a matter of personal preference from what I have learned. Some people believe the foliage should be left alone during the winter months and I must admit that is the easiest solution when there are so many other plants that need to be cut back. Then there is the school of thought that cutting back the foliage to three or four inches removes any insects that may be lingering in the thick leaves and eliminates the threat of them boring down into the rhizome.
Although I often leave the foliage alone, this year I thought I would cut it back on many of the new iris to see if it invigorated them for next year. I guess we'll just have to wait and see.