Fall is traditionally a time for cleaning up gardens. Normally, we recommend clear-cutting dead stems to help control insect and disease problems. With herbaceous perennials that have been pest free, you might want to consider leaving some to provide structure, form, and color to the winter garden. For example, ornamental grasses can be attractive even during the winter months. But those near structures should be cut to the ground because they can be a fire hazard. Perennials with evergreen or semi-evergreen foliage can provide color. Of course, some perennials are naturally messy after dormancy and should be cut back in the fall.
Foliage can be left for other reasons. For example, foliage left on marginally hardy plants such as tender ferns helps ensure overwintering of plant crowns. Also, seed heads on some perennial plants can provide seed for birds. (Ward Upham)