Why even try to turn it off. If you try to use it for a dml statement you will recieve.

bitbom:/home/oracle> oerr ora 1552
01552, 00000, "cannot use system rollback segment for non-system tablespace '%s'"
// *Cause: Tried to use the system rollback segment for operations involving
// non-system tablespace. If this is a clone database then this will
// happen when attempting any data modification outside of the system
// tablespace. Only the system rollback segment can be online in a
// clone database.
// *Action: Create one or more private/public segment(s), shutdown and then
// startup again. May need to modify the INIT.ORA parameter
// rollback_segments to acquire private rollback segment. If this is
// a clone database being used for tablspace point in time recovery
// then this operation is not allowed.