With the release of Fire & Blood and its unique depiction of the Targaryen three-headed dragon on its cover, how standardized were sigils historically? Were they like modern brand logos or were they more of a basic description that artists and craftsmen were free to interpret as they wished?

Quite standardized through the various Colleges of Heralds. 

The whole point of heraldry was that an insignia should be instantly recognizable proof of identity for men in armor. This was important for a number of reasons: making sure you got taken prisoner and ransomed rather than killed, making sure you don’t kill a friend or family member accidently because you didn’t recognize them in full armor (something that happens all the time in Arthurian myths when someone dresses as a mystery knight), making sure your soldiers know who to follow on the battlefield if they can’t see the banner, etc.