I recently visited Maastricht ATC, and one of the controllers told me that he'd been told during training that if he had to make an A/C turn by more then 10, he had probably made a mistake earlier.
Depends on the geometry of the airspace and the confliction. That said, if any 'instructor' is limiting his trainees to turns of only 10 degrees, he's painting himself and his trainee in to a corner.