Nick,
you need to do a Full backup of the IS for 2 things -
1. This is what you need in case of a disaster recovery, or restore because of corruption to the IS.
2. The full backup when completed successfully, when purge all the commmitted logs.
Do mailbox backup as an extra, and make sure it is always last in your backup job.
Or don't know it at all and follow good admin practices like..
1. Turn on deleted item retention, set it for 30 days - that way if a user deletes an email, they can get it back themselves.
2. If you need to delete a mailbox, export it to a PST first.
Mailbox backups are a pain to configure, backup, restore, and just keep in check
Hope this helps
---Cheers
shirmal