I only consider the books as canon myself (because I haven't seen the movies! ;D), but there is an argument to be made for the movies being canon too. I mean, in other fandoms you can often see fic marked as either "book canon" or "movie canon" (never seen that in HP for some reason) and if the fic was written with the movies in mind… well, then, you could say the movies are canon for that fic!