I leave my plants alone until the stems are pretty shriveled up by the cold. Any green stems are feeding the roots for winter. Cutting short early reduces stored food in case of late Spring or hard winter. I do trim off the seed pods before they produce seeds. Puts the energy into storage not seeds. I also leave a bit of stem on top of ground so I can locate plants in spring. My plants sprout so late that I might think they were dead or lost without the stem markers.