When to Apply Mulch for Wintering Lavender

We have a farm in Wisconsin, USA and have 30 lavender plants in my garden. I had only a few last year and most came back after a normal winter and we added more plants this year. We had covered them with straw last year. My question is, should we wait until after they freeze before covering them or should we cover them before they freeze? We usually get a good snow cover over the straw. Also should we wait until after there is no danger of frost before we uncover them?

Wait until the first few frosts before adding the straw cover, in order to make sure mice have found other winter quarters already. A mouse under the straw is going to chew everything in sight and you will likely have no plants left by spring. Even if they don't eat the plants, they need to use their teeth to wear them down to manageable size.
Remove the straw from the immediate vicinity of the plants as soon as possible in the spring, but retain some mulch on the soil all year long.