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.