Liz, I always use fixatives on my charcoal and graphite pieces, it works fine with them. But ask anyone who does pastel work for a living, none of them endorse using fixatives on a pastel portrait because of what it does to the colours over time. Pastel will always fade slightly over time, but fixative speeds that process hugely. Plus it hardly does a thing for pastel. It will still smudge as it would without fixative, all fixative does is still the top layer of pigment temporarily to the paper. It will still flake off over time.