Thank you for your question about cutting back and mulching your perennials.  The OSU Extension publication entitled, “Winter Care of Perennials” states: 

“Once your perennials start to lose their leaves, die back and go dormant, you can go ahead and cut them back in the late fall or early winter. By cutting them back 6-8 inches above the ground the stem will be able to hold now in place which helps to insulate your plants.  You can also wait until spring to cut them back if you prefer, however mulch is easier to apply if they are cut back.  Either way you’ll want to mulch your perennials with 2-5 inches of mulch for the winter.  The mulch can be any kind of mulching material such as hemlock mulches, dried grass clippings, etc.  It serves the dual purpose of keeping in the winter moisture and acting as insulation for the root system.  Our perennials risk losing their root systems from tissue damage with our continuous freezing and thawing conditions.  It is best to wait until the ground has frozen lightly before applying mulch, this insulation will keep the ground frozen.” 

There are perennials that prefer no additional mulch through the winter.  A list of these plants is provided in the publication:

http://extension.oregonstate.edu/deschutes/sites/default/files/Winter_Care_of_Perennials-reference_list.pdf 

If you need additional information, you can call the Deschutes County OSU Extension office in Redmond at<personal data hidden>.  You can ask to speak with Amy Jo Detweiler or Toni Stephan, members of the Horticulture Staff.  Both Amy Jo and Toni welcome your home garden questions.