Ken,

In my opinion, relief valve discharge loading should be treated as occasional as :

1) Lexically speaking it is occasional.
2) It is not secondary as it is not displacement driven and I don't see a "seismic anchor movement" type scenario in relief valve loading.
3) Regarding fatigue , if you envision that as a problem and since it is stress controlled fatigue, ty to keep the computed stress less than endurance limit.

Regards