We have tags for some other BitTorrent programs1, such as deluge, ktorrent, transmission, qbittorrent,utorrent, and--though it's only on a handful of questions and I don't think I know anyone who uses it anymore--vuze.
It looks like we also have a number of questions about those programs that don't have those tags. Assuming we do retag posts with rtorrent, we could add appropriate tags to some other BitTorrent-related posts as well.
I use qBittorrent and the qbittorrent tag has helped me search for questions and answers. In at least one case, I believe that noticing the tag on a question led me to read the question and post an answer, though it's possible I'd have noticed and answered anyway. I presume the rtorrent tag will confer a similar benefit. It's not as popular as several other BitTorrent programs but I think it does see significant use.
I'd say yes, we should add the rtorrent tag to posts it applies to, including old posts.
1: It's common to call them "BitTorrent clients" but I use the more awkward-sounding term "BitTorrent program" because, from a networking standpoint, these programs behave both as a client and as a server. When BitTorrent was new, some people called them "servents" (server/clients) but that terminology doesn't seem to have caught on. (The other distinction for BitTorrent is between client and tracker, but many BitTorrent "clients" have the ability to operate as a traditional centralized tracker, and almost all of them support distributed tracking, usually via DHT, which is more significant today than centralized tracking.)