I don't think it's needed. We can leave it for now, but writing the hash before calling the data is extra work.

But more importantly, I think the spec is just going to be using a single hash function throughout the code. I don't really see a reason to introduce multiple. If you can make a good argument for why we might end up using both sha256 AND some other hash in the project, I'll be convinced. But if the project as a whole just has 1 hash function, I don't see the point.