My rule is, don't use a dependency to implement your core business. Is JSON parsing our core business? No, so why would we ever write -- and thereby commit to supporting for its entire lifetime -- JSON parsing code? All the code you write and support should be directly tied to what you as a business decide are your fundamental value propositions. Everything else you write is just fat waiting to be cut by someone who…
Well, one special edge-case would be where you only need to parse some extremely tiny subset of JSON (for example: you only need to parse dictionaries whose keys and values are positive integers, like {1:2,3:4}). Then, depending how expensive the full json parser is, it might be worth your while just writing the limited parser yourself. Of course, you might say, inevitably feature-creep will expand the list of things…
You don't know what your requirements are. The customers haven't told you yet.
If you pick a library with a straightforward interface, especially one that isn't too opinionated, you can always drop in a custom implementation later on. Frameworks, not so much (but that cuts both ways; the people who will write libraries often love writing frameworks too)