Hard to recommend the 10.5.5 update just yet. There seem to be some installation issues. Half of the machines at Boyceworld took the installation without incident and are working fine. The other half took forever to restart after the install, then went to a blue screen, then turned themselves off. After restarting a few times all seems normal but everything leading up to that certainly wasn’t pleasant. I think you can live without it, but if you are daring and want to install 10.5.5 at the very least be sure you are completely backed up, AND repair permissions before you install, AND be sure nothing is running when you do the install (hint: restart first, then do the Software Update), AND be sure that all external disks are disconnected.
Personally, given all that, if you still want to do 10.5.5, I think you should have me do it. And use the 10.5.5 Combo Update (click this link to get it).