ribuck:
Accuracy/precision is a red-herring unless you're treating numbers as strings, since JSON-RPC numbers ARE ALWAYS double-precision floating point numbers (according to the JavaScript/ECMAScript spec). Bitcoin could send a number that looks like 2100000000000001, but the code that interprets that JSON-RPC number will convert it into an inexact double-precision floating-point equivalent. And then the code that displays that number will have to decide how to round and format that inexact floating point number and display it to the user.
When we need more than 51 bits of precision (wouldn't THAT be a fantastic problem to have!), then we'd HAVE to send numbers as strings, and have the JavaScript (or whatever) on the other end feed them into a bignum-type library to handle them.