Portage typically unmerges all but the newest kernel sources, while I'd prefer to leave the sources to the running kernel and backup kernel installed. I don't reboot very often and I don't bother to build an updated kernel until I'm ready to reboot, just in case there are any issues. I also like to keep the sources to the previous kernel around just in case I need to revert.
A tip posted on the Gentoo user mailing list in 2009 by Mike Kazantzev shows how to accomplish this:
First, define a kernel set in /etc/portage/sets.conf:
Tthen add the @kernel set to your /var/lib/portage/world_sets file.
By doing this, you will need to manually unmerge the specific kernel source packages you want to remove, but none of them will ever be removed otherwise.