One of the best practices for installing RVM (BTW: one of the greatest tools available when working with Ruby) is to install it as regular user (and not as root).
But if you try to install a new Ruby version via, e.g.
rvm install 1.9.3
you may get a prompt for your root password, since rvm is trying to update your operating system.
To avoid this, simply disable the autolibs feature of rvm:
rvm autolibs disable rvm install <whatever version you want to install>
However, you should make sure that your system is up to date by running
apt-get --quiet --yes update