What to Do with Perennial Herbs in the Fall?

A couple of years ago we have purchased from you a good quantity of herbs, some of them flourishing beautifully, some of them not so. Therefore, I would like to ask your advice regarding the perennials, like for instance SAGE - THYME - LAVENDER - MARJORUM - OREGANO etc..
In the fall, after I have harvested what I needed during the summer, do I trim them all down or do I leave the remaining growth ?

It is better not to cut them all the way down. Many herbs can survive aggressive pruning in the fall but some cannot. To be safe just trim back the unsightly growth, but otherwise leave the plants as they are.
Later when the ground freezes but before the first permanant snowfall, it is a good idea to mulch your plants with straw, dead leaves, etc.