I like waiting till a bow is otherwise finished ,or at least tillered out to about half it's intended draw length, to put the first few layers of sinew on. Because as the sinew drys it will pull in some reflex,especially if ya tie it backwards an inch or so ,then sinew it. So I'm in favor of sinewing a finished bow to increase the draw weight. Scrapping off a finish isn't hard, just remember to degrease the back too.
Ralph