Answer: It is definitely a good idea to mulch around the plants with something, chopped autumn leaves are truly great as would be any natural mulch material such as shredded bark or bark chips or half finished compost. You should have a layer of mulch year round, a summer mulch is usually about two to three inches thick. Winter mulch is a bit deeper. The mulch rots down over time and helps feed the soil, in addition to its other benefits.
Do not cover over top of your perennials however as the mulch can smother them. In mid winter, snow is a great insulator. If there is no snow and temperatures are very low, you could cover the perennials with some evergreen branches since these do not pack down or hold moisture. Remove them in early spring once the most bitter weather is past.
Roses may be a special case. If you are growing winter hardy shrub roses (such as landscape roses or rugosa roses) rated for your zone, a generous mulch over the root zone should be sufficient. This is assuming any grafts are well below the soil surface -- or (preferably) they are on their own roots.
Hybrid tea roses however will most likely need special protection such as a mound of soil (dig it from elsewhere in the garden) about a foot high piled over the base of the plant. Do this in November, then gradually remove it in the spring.
Your zip code places you in winter hardiness zone 5A, the coldest part of zone 5. Depending on the microclimate where you are gardening, it may be as cold as zone 4. This is why I mentioned winter hardiness. As long as you have planted plants considered hardy to zone 5 -- and even better to zone 4 -- you should be in good shape.
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