What is the proper way to sand live edge piece of wood in preparation for a polyurethane finish?
The reasons to sand before applying finish are to scuff the surface to give the finish something to cling onto, clean the surface of any dirt in the fibers of the wood and to flatten any imperfections.  Poly will generally stick to a good clean surface that is as porous as wood.  I would just go over it with a wire brush to make sure it is clean and evenly scuffed.