According to the Gentoo docs http://www.gentoo.org/doc/en/faq.xml#distfiles, this is safe (although I should pgrep emerge to safely delete the work cache).  It's not the standard way to manage distfiles though, the eclean tool cannot remove distfiles for currently installed ebuilds.  If we use eclean, then when we upgrade a target, we can clean out the old sources for whatever was upgraded.  I'm torn, on a device like the Pixel, I'd rather not keep the distfiles around, but I'm sure what the right default is.