I tend to leave my herbacous perrienals old growth to protect them from frosts and discourage them from re-generating until springtime.

I agree with Jimmy, I leave them and then in the late spring I'll cut them down to about 6-10inches. Any cutting material will then hopefully root.

Agree with Jimmy & Eileen, better to wait until late Spring. The general guideline for trimming is to only remove about ⅓ of top growth. This is to prevent severe shock. But you can trim again later in the season if need be. I started using fresh sage when making lemonade. It's a hit with the folks.

You can cut them back very hard (down to 6 - 10 inches as Eileen says) once all danger from frost is past.