You can safely empty some directory`s in Gentoo. But as always – first make sure if my suggestions are OK for YOU too!
# As superuser
gentoo ~ # emerge –sync
gentoo ~ # emerge -av gentoolkit
gentoo ~ # emerge -av –newuse –deep –update world
gentoo ~ # emerge -av –depclean
gentoo ~ # revdep-rebuild
gentoo ~ # rm -rf /var/tmp/portage/*
gentoo [...]