How do I remove cached .deb files?
I think it's safe to remove the archives files. So if you want to free up disk space this are my recommendations:
To delete downloaded packages (.deb) already installed (and no longer needed)
sudo apt-get clean
To remove all stored archives in your cache for packages that can not be downloaded anymore (thus packages that are no longer in the repository or that have a newer version in the repository).
sudo apt-get autoclean
To remove unnecessary packages (After uninstalling an app there could be packages you don't need anymore).
sudo apt-get autoremove
To delete old kernel versions
sudo apt-get remove --purge linux-image-X.X.XX-XX-generic
If you don't know which kernel version to remove
dpkg --get-selections | grep linux-image
Source: Limpiando Ubuntu: comandos y programas (actualización) & How do I free up disk space?
Yes, it is generally safe to remove them, unless you had a bad Internet connection and you needed to reinstall certain packages.
To remove them, open a terminal and type:
sudo apt-get clean
You can also use Ubuntu-tweak and Bleachbit.
And related at the clean cache... you can use this command
sudo dpkg --purge `COLUMNS=300 dpkg -l "*" | egrep "^rc" | cut -d\ -f3`
in order to delete all configuration related to unistalled program.
And:
1) localepurge: for delete localizazion packages not interested for you
2) deporphan and his gnome interface gtkorphan.