Alltough I never head problems with lost database connections while writing CDRs the scenarion may occure. In this case asterisk would not be able to save the record.
If correct storage is a must I would store the CDRs locally and transfer them lateron to the billing server. The easiest way to achieve this is indeed database replication. The replication configuration itself depends on the databases involved (However for a real database replication the dbs should be of the same venndor in the same version - eg. both MYSQL 5.1).