Experts and BHG readers answer.
For most perennials it makes little difference if they are cut back in fall after they have frozen back, or in late winter. The key is to wait until they die down naturally. Avoid pruning in early fall, because that can stimulate new growth that will freeze off over winter.
If you have diseased plants, it is a good idea to cut off the diseased portions before winter to avoid carry over of the disease to next season.