You can cut it now or wait until spring. As far as the wood goes it doesn't matter. You can remove the bark and most of the cambium now with a little work or you can wait until spring when the bark and cambium peel right off with very little effort and you will have a pristine back for your bow.
Knowing the life cycle of plants and trees really helps when making wood bows. If I am cutting fresh bow wood I do it at what I consider the appropriate time. If I'm using precut wood, no matter when it was cut I make bows with that too. It is your choice!