By this point, it was very common for scholars to write inscriptions on paintings, either ones they had just seen painted, or old ones in their friends' collections.  Therefore, painters could have anticipated that empty space would later be filled by inscriptions.  However, it is not the case that all paintings were inscribed  until all empty space was filled, so an artist could not assume the addition of inscriptions and would have to come up with a composition that would work whether or not inscriptions were added later.