When that JSON payload changes (intentionally or not), you will run into a mysterious problem in some unrelated area of your code. It will be significantly more expensive to fix than failing fast at the point of parsing.
But depending on the situation, that problem may never happen. I'm not a big fan of introducing complexity to guard against code screwups in the same codebase.
The hardest bugs are the rare bugs. Common and frequent problems are easy bugs to fix. These subtle or rare changes are those which should be feared. If you model only that which you support, finding the source of the problem becomes much easier.
