Last stable release is always advised in Production.
Today, it is 5.1.5.

Using release candidate, you may face some issues.
This may not be blocking to some users, who are able to post issue in the forum and manually apply patch in code when provided (if required), but without committed response delay.
Other option to quickly migrate to last version, in release candidate, is to subscribe to Support Service. This way, you can get a patch to be deployed (no manual update of code required) if you face an issue, with committed response delay.
NB : Support Service is a good solution to secure your use of ProjeQtOr 😉