You should update statistics for all tables and for all databases every night.
According to a SQL Server tuning expert at Microsoft (one of their best), it was suggested to me that if your SQL Server maintenance window allows for it, that you should update statistics for all tables and for all databases every night.
This is because the auto-update option is less than perfect, and accurate and up-to-date statistics is so important to overall performance of SQL Server. Of course, if this is not possible because of time constraints, then you will have to perform this step less often, or even not at all, and rely on the auto-update feature. But if you do have this window, you should take advantage of it.