There still is a slim chance that a package you already downloaded will be updated again in the repos before you run yum again.
In that case, yum will download the new package and you can end up with extra package files on your drive after the update completes. If that happens, then a yum clean all
will take care of it and delete the remaining package files. (Do NOT run a yum clean all
before completing the downloads and the packages get installed or else you will have to download all of the packages again)