Sometimes features go away permanently. For instance, if you like Gnome 2 or the Gnome screensaver (my favorite is Cosmos), stick with 10.04.04 until support runs out.
At other times it depends on your hardware. For less than 640MB RAM or no 3D Graphics, get the latest release of Lubuntu or Xubuntu, and keep ubgrading every six months. As long as the lightweight desktops run on an old machine, the more new features, the better, especially with LXDE/Lubuntu.
But with 768MB to 1GB RAM, you will need Gnome 2, not 3, stick with 10.04.04. Gnome 2 is still on 10.10 (possibly also as Ubuntu Classic on 11.04, depending on how you installed/upgraded), but support runs out earlier.
Whenever you are not sure, either use a live CD to test all features you use, or try an install on a replacement hard drive. You can keep your data and settings by cloning the old hard drive first, then upgrading (safeguard the old hard drive in case you want to go back).
If hardware constraints or preferences for Gnome 2/KDE 3 are not an issue, go with your gut. Upgrade every six months if you love to try out the newest and latest software. Upgrade every LTS release if you prefer not to do unnecessary work on you computer.