I can't comment on the statistical risks as there is no quantitative data to offer such calculations. From my understanding, Firmware version 1.05x and 1.06x are identical except for a minor change to FTP. I have not read of any accounts of data loss resulting from an upgrade.
Regarding data backup, you should always maintain a backup of your critical data on a separate physical device. Also, if your DNS-343 is setup using a RAID or JBOD configuration, RAID provides data redundancy, but is not a substitute for true backup.