What Ajez means is this:
Usually when updating SL we advocate that you download the new iso burn it onto a dvd and use what is known as Sixth Sense. This analyzes your current install and will just update the packages that need updating.
Technically speaking, you can just keep up to date with world updates, however, unless you know how to solve portage problems and broken things, a world update in SL is not advisable. Why you ask, well it derives in part from the SL philosophy. Without getting to into the Gentoo Handbook, Gentoo takes the default that ONLY stable packages will be installed. SL on the other hand (and unless you tweak your package.mask file) will install the VERY LATEST version of a package. So with a world update, portage will upgrade EVERY PACKAGE ON YOUR COMPUTER TO THE VERY LATEST PACKAGE. While that may not be a bad thing, sometimes that can lead to instability and breakages with things like KDE or system packages
You can even update to the latest version but leave all non "system" packages alone, by doing an emerge -uD system (I think). So the following command would leave things like OpenOffice alone, but would upgrade KDE, your kernel sources, modules, that sort of stuff. Sorry if this got confusing. The idea is with SL just wait for the DVD and use sixth sense, it's just got fewer headaches associated with it.