When working with the cold wax medium or even with acrylics, a good cardstock is one possibility for cutting out your stencil. However, I also use thinner weight papers when working with the cold wax/oils - my favorite is newsprint. I've also used freezer paper or palette paper. Any paper can be used; the question is what kind of effect you want. A cardstock stencil can also be used to apply paint with, whereas one made of newsprint would not be as effective due to how thin it is.