I would suggest complying with the published limitations for your aircraft...read the POH, different cessnas are different, but I do not recall a limitation on slips with partial flaps, only full flaps. With that being said the story about flaps blocking airflow and stalling the rudder is an old wive's tale. The real issue was turbulent airflow causing buffeting, which would probably be harmless, but might scare or confuse an inexperienced pilot.