For production systems, it would be recommended to backup the OpenAM configuration data nightly, specifically ou=services,o=[your organization]
I'm not too sure about ou=opensso adminusers,o=[your organization], but I'll backup a copy just to play safe.
Operationally, how huge will this OpenAM configuration data be? I can safely say it's very small. Will it have any performance hit on the OpenDJ server? Not likely. If the OpenDJ server is embedded which shares the same JVM as OpenAM, will it affect the functioning of OpenAM server? Not likely.
Anyway, we are talking about backup during off-peak hours, ain't we?