I have set for one of the issue type schemes the default issue type to be a sub-task, however, when creating a sub-task the default is not applied.
Once the correct sub-task is selected JIRA then caches that selection and will remember it until JIRA is restarted.
It would be nice to be able to have a default Issue type as well as a separate default for sub-tasks (a whole other request in itself).
There was a point in time where this was functional in our environment, however, it is no longer.