Craig Porteous explains the types of things you should document in SQL Server Reporting Services:
If you’re using Kerberos authentication with Reporting Services you’ll at least have to update the rsReportServer.config file with the correct authentication mode. Beyond that you have SPNs on your SSRS domain Service account to consider. This may be managed by your domain administrators but it’s still a good plan to back these up or check that someone is backing these up. It is too easy to clear SPNs or constrained delegation settings accidentally.
Having a backup will also give you a good reference point for building new environments. You can of course easily see SPNs from the command line using this command:
SETSPN –L DOMAIN\SERVICEACCOUNT
What this boils down to is, if you lost your Reporting Services installation today, could you get things back to the way they were?