Before an upgrade, you should install all updates.
But I advise against the upgrade! It makes things more complicated, because some packages are entirely different in new versions. Moreover, the risk of data loss is very high.
You can consider renewing the installation; it will keep your data and even personal settings. According the documentation:
Renewing the Installation without formating the partitons (in contrast to upgrading), will also keep the personal data and configurations under /home but will renew all system settings under /etc as well as the default set of installed packages.
Before renewing the installation, be sure that the /home directory is on a seperate partition; if not follow the procedure to do so.
I upgraded 11.04 to 11.10 last year. Everything was OK, but the performance loss was sensible; for example, shutdown was taking more time (sometimes twice and sometimes four times more). Some days ago, I installed the new 12.04 without formatting the /home partition; I was surprised when I saw even my desktop backgraound was not changed.
I just needed to reinstall my apps; but not my preferences and data for those apps. For example, when I reinstall Tomboy Notes all of my notes appeared automatically without any backup or doing anything.