Is there a way to automatically transition a task issue once all of it's sub tasks are completed?

Hi Jeremy, I use an add-on called "JIRA Misc Workflow Extensions" to do this.

Use Case:  When all child issues are closed, automatically transition the parent so users won't have to do it manually.

Implementation:

In the parent issue's workflow, add a Post Function, which will only be used by this automatic process.

• Add the "Hide Transition" Condition to hide the button from regular users.
• NOTE:  This transition cannot have any Screens or required field Validators.
• Add a "Sub-Task Blocking Condition."  This prevents the parent from transitioning until all children are closed.
• Example:  All sub-tasks must have one of the following statuses to allow parent issue transitions:  Closed

In the child issue's workflow, add a "Transition Parent Issue" post function to all "Close" transitions. 

• Example:  Transition X will be triggered on the issue's parent issue.  (X is the ID of the transition in the parent workflow or the transition's name if it's unique.)
• Move the post function to the last position in the list. (This rule must run last.)