> I'd sooner criticize a parser for mangling numbers above 2^53 (also allowed by the spec)
The spec has something specifically to say about this[0]:
> Note that when such software is used, numbers that are integers and are in the range [-(253)+1, (253)-1] are interoperable in the sense that implementations will agree exactly on their numeric values.
Basically, allow up to 2^53, and you ought to be fine.