In this instance, all of the extra buildings tagged as amenity=self_storage should only be tagged as building=warehouse. I only tagged the storage facility's main office with shop=storage_rental.

I wasn't aware of the standard put forth on the wiki when I tagged them. I had previously thought the standard was placing the shop=storage_rental at the storage facilty's rental office while tagging extra buildings with amenity=self_storage.

So at least in this instance, I can remove all of the amenity=self_storage tags. 