There is no "real" risk of data loss during the 1.07 to 1.09 upgrade - but - as with any upgrade, things can go "bump" causing the data to become inaccessible, if it's your only copy of the data I would strongly suggest a backup, if it's a backup of your data then you have the data available elsewhere should things go awry, so I wouldn't be too concerned.
Mind you - it is possible to have two things go wrong simultaneously, so there is some risk of data loss - and if it's any consolation, I use my DNS-323 as my backup, and I have not had any issues with data loss using it as such - I have not had any disk failures with my primary storage, I have had a disk fail in the DNS-323, and to be fair, the disks in it were not new when they were installed.