Unless things have changed in Maemo 5, the application manager will automatically do apt-get clean for you. It's only when you're doing manual operations that you could end up with files in /var/cache/apt/archives/
that can be removed with apt-get clean. It is, in any case, a safe operation though. The only reason to leave files there would be if you wanted to re-install something without having a network connection (it could read the .deb from the cache).