There are couple interesting notes on Metalink:
GATHER_FIXED_OBJECTS_STATS Considerations [ID 798257.1]
Also, Metalink note How to Gather Statistics on SYS Objects and ‘Fixed’ Objects? [ID 457926.1]
mentions that “statistics gathering should be done when database has a representative load so that the statistics reflect the normal database activity”
There are some fixed tables that are simply very volatile by nature and it will be extremely hard to get accurate statistics on.
Still, better with than without.
The place I would look is how much time is spent for recursive sql, which basically touches the dictionary tables.
So how often to gather??
It depends. It depends on the load profile, if you regularly add more users (concurrency) in your application,
then gathering system statistics shall be in accordance to that.