This truly is one of those where it depends on what we're trying to accomplish. If we're just trying to show additional networks. Then on a router we would add loopbacks. This make as an effective away to create multiple networks even when not connected to 50 additional networks.
If we're using serial interfaces with frame relay, then we normally use sub-interfaces to differentiate the network connections on a single physical connection. The subnet mask that you'll see in our recordings, if not being used for the show is normally because we're pre-configuring it for another demonstration solution and we've just chosen not to mention on the episode.