Teasing the crown first can work but it depends on your hair type and how tight you want the twist to sit. On fine or slippery hair, backcombing before you pin just gives you a fuzzy lump that wont hold and you lose the smooth shape that makes a French twist look clean. I get better results backcombing after the twist is pinned, just at the crown, so you can control how much lift you add. Doing it before means you are guessing at the shape before the hair is even in place.