I\'ve heard that JSON serializes all the data, which stops me having problems client side in terms of cross-browser support etc..
I\'ve been using AJAX with jQuery and i
I think you are confusing the terms.
AJAX stands for Asynchronous Javascript and XML, which is a mechanism used to launch asynchronous HTTP requests to a server using JavaScript. Don't let the name fool you; there's no restriction on you only retrieving JavaScript or XML from this technique. You can quite happily return other data formats as well (HTML, plain text and JSON, to list a few).
JSON is just one of these formats. It's a data interchange format, where-as AJAX is a technique to communicate with a server after the initate page load has completed.
To answer your question on whether you need to specify the dataType
; jQuery will best guess the response format (be it HTML or JSON etc), so you're usually fine to omit it.