I have two appletv's. One is set to sync x most recent items (in my case movies). The other is just set to sync items that are checked off. Using the sync most recent items setting, any time I add an item to itunes for which I've set this setting (ie. a movie or tv show), the appletv automatically syncs (I don't need to click the sync button). The one where I did not select this option for movies and tv shows does not auto-sync.
If you don't want to have a lot of things sync, but want the appletv to auto-sync, i'd suggest setting this to sync the 1 most recent movie and tv show, that way whenever you add a movie or tv show it automatically syncs. If you're limited on space you may only want to do this for movies.