The best practice is not to assign milestone tasks to resources. That leaves
it to the Project Manager to determine when a milestone has been achieved and
to mark it complete in the schedule.
We have been assigning a resource to milestone tasks, and our work-around is
to use the arbitrary "0.1 Hrs" to complete the milestone task. This allows
our resource to complete the task, and they simply enter.1 Hrs to complete
the milestone.