Coaster, I like to have a bow to at least floor tiller stage before adding the sinew. I have, however, sinewed an already tillered bow, that was intended to be sinewed, with good results.
By getting it to at least floor tiller stage you can see the sinew working at drawing the bow into reflex as it dries. Some re-tillering will be necessary in most cases after the sinew has cured out completely but if you are careful about adding the sinew equally this will be minimized.