In the USA (based on your callsign) I know the FCC specifies /KT, /AG, /AA and /AE because they are in the US callsign group. According to the FCC (97.119):
"If an indicator is self-assigned, it must be included before, after, or both before and after, the call sign. No self-assigned indicator may conflict with any other indicator specified by the FCC Rules or with any prefix assigned to another country."
So it sounds like technically you could argue that you should never sign /R (for repeater), or /M (for mobile - since M belongs to the UK) or /MM (for maritime mobile - since MM corresponds to Scotland).
The consensus here:
is that the FCC rule is badly written. If you're really concerned, you could sign /R. or /RPT, since that is not a prefix assigned to another country.
Based on the ITU article 19, I think you could also make the argument that R is not a prefix - that R0-R9 are prefixes. (http://life.itu.ch/radioclub/rr/art19.htm 19.68). After all, a foreign station in the US can't just transmit VE1RAC/W, they need to transmit VE1RAC/W7.