My understanding is that it won't be secure if you aren't using SSL when the Tableau Server communicates outside itself i.e. to Desktop or another server.
If you are having the rserv in the same network environment as the Tableau server, you may not need to worry much. But in case you need to secure, you may use HTTPS configured to communicate with the rserv through the network.
Thanks guys, is there any documentation for configuring Tableau to connect to Rserv via https? I had Google searched this but nothing was an obvious match, so we weren't sure if it was possible.