You can stream to the computer and they won't take up space on it either. But this is something you have full control over.
i.e as long as you do not download them to the computer, they will not take up space on it.
You can stream them directly on the Apple TV from the iTunes servers. But if you want to stream them from your computer, then they will need to be downloaded to the computer to be able to stream from there to the Apple Tv.
Other than that, if you are so concerned about space, perhaps an external drive for the movies may be useful.
Also note, that the movie rights holders can pull the movies from iTunes at any time, so you cannot count on them being available for download always or forever.
Unless you plan on having hundreds of movies, its a good idea to put them on an external drive.