The way browsers implement both JavaScript and document rendering via a single-threaded execution queue, that isn't an issue. Once the page begins rendering, callbacks won't have a chance to execute until after the document is rendered and ready. That's the same mechanism that makes the setTimeout(fn, 0) trick work, for example.

It feels a bit like driving fast with no seat belt on, but it's safe.

If you do want to be extra cautious, MBlume's suggestion to wrap the success handler in $(document).ready() works fine too. That's effectively the same as not wrapping it.