Matthew
Theoretically, you should backup the System State and the volumes if you want to, in case a disaster occurs, and with Exchange backup up the information store is enough, because, thanks to GRT, although you can't backup individual items, you will be able to restore them.
The best choice would be to backup Exchange first to disk, then duplicate to tape. If Exchange backups are only on tape, any restore needs a temporary restore to disk of the entire Information Store (even if yoy just want an individual mail!).
But if you look through this forum you will see there are so many issues having to do with GRT. In my case, I complain because GRT doen't recycle disk backups properly, causing a potential lack of disk space if you don't do it manually. And there are many more issues.
Regards