At $work we have a job that runs every night that calls clusterdb  on our database. clusterdb reruns cluster  against any tables that have previously been clustered.
If you want to know which tables have previously had cluster run on them, the following query does the trick:
SELECT pg_class.relname, pg_index.indisclustered FROM pg_class JOIN pg_index ON pg_class.oid = pg_index.indrelid AND pg_index.indisclustered = 't';
This will list all the tables that cluster will reorder.