You can safely remove the packages in the /usr/portage/distfiles/ directory.
That directorty holds all the sources you donwload and so if you delete any of them you have to download it again if you want reinstall/modify the package ...
To a better way to this check
http://forums.gentoo.org/viewtopic.p...er=asc&start=0
Its also safe to remove the files in /var/tmp/portage/ and in /var/tmp/portage-pkg/ as these dirs only holds temp archives of the compillation process ..
Note that all this info was learned from the gentoo forum and take a look at it as syg00 said is always helpfull ...