On Ubuntu and Debian you should put a file with the bits you want to add into the /etc/sysctl.d/ directory (see the README file there). This means you are adding your own file, rather than editing a system one which an OS update may wish to change later.
And you can set forwarding per-interface, rather than globally. Probably doesn't matter on the Plug, but as a principle it's best to only switch on what you need.
I have an /etc/sysctl.d/60-My-eth-routing.conf