I would not waste it for Wishes. It gives you a good spot to watch the show. For Fantasmic - we always got in without any package. But if you want to guarantee that you get in without having to wait on a line then it is worth it. The Fantasmic theater is huge and going in February and August we always got in and we went during busy times.
The other problem is, right now, you cannot get your 4th fastpass until you use all of your 3 fastpasses. So if you use one of the 3 for the late shows - as I believe you cannot get your 4th FP. But I bet that will change.