BPM and attachments

I have a question with regards to BPM and document attachments. We have many scenarios in which documents would be attached to the process instance for review etc. Are there any best practice guidelines for storage of such documents in a BPM process. What is typically the best place to store the document after the process instance is closed? Should these be sent to the back-end application or stay in BPM even after the process instance is closed. What options are available? Any guidance would be much appreciated.

Thanks,

Gaurav