Is it necessary to back up data before installing Windows Vista Service Pack 2 (SP2) from online updates?
No, it is not necessary, but it is recommended.
Service Packs are many updates all packaged together, and it only installs the updates in the SP that are not already installed or up-to-date. However, there is always the chance that an update can mess up your computer, etc, and with a SP, you should backup, just in case.
You should always back up important data before critical system updates. That's what they tell you anyway. It's better to make a complete system backup in case you are left with an unstable system rather than backing up just single files.
That said, I think that 99% of people do their service pack updates without backups and it should be fine.
Yes, this site and other forums are full of posts about lost data for one reason or another.
You should Always have a recent backup of your important data.
In addition to a backup of your data you could make a system image of the hard drive before you install a Service Pack as suggested by member slhck.
When windows installs a service pack it automatically creates a Windows Restore Point, which can be useful to roll back the service pack install if things go wrong, but should not to be relied on as a replacement for data backups or system images.