I typically generate a random unique code. Depending on the situation, I might append it to the device ID to facilitate support. The key is to make it so that the user can easily unlink with a factory reset or by uninstalling the channel (i.e., by clearing the registry).
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)