Many take the prefix pre- to signal a redundancy, but it tells us that something is done before something else. It’s fine to preregister and preheat.
If you preview a movie or play or exhibit, you are viewing it before it's more widely available to others. We don't object to this construction based on the argument that "view" accomplishes the same thing.
We don't take "preheat" to mean heat before you heat. It means heat before you proceed to the next step. "Heat the oven" might do the trick, but "preheat the oven" is more clear. "Preregister" means you register now and not at the last minute.