First, Needles are mostly used in FSC since they are the best weapon against Duke body (and very good against trojans). On Duke, dps-wise if you have 4 Needle, you will do more damage with 1 Plague Needle and 3 Blitz Needle. But as soon as someone doesn't have a Needle, Blitz Needle > Plague Needle (meaning, 3 Blitz Needle will out-dps 2 Blitz Needle + 1 Plague Needle). I didn't do the calculation, you'd probably be able to find it by searching the forum.

That's only dps-wise, because Plague Needle poison also reduce attack of mobs by half. That's a valid reason to use it.

Outside of FSC, unless you specifically want some poison (and in that case, i suggest you look into another poison weapon), you'd better go Blitz Needle, since it OS pretty much anything (dead enemies can't damage you, so you don't need poison in the first place).

As for RJ specifically, I'll let someone else answer, because i'm not sure about it.

Edit : as for RJ, I would use either a Venom Veiler (to be SURE to poison it) or a Biohazard (good damage, good mobility, poison, what else ?) instead of a Plague Needle. Just because I don't like using piercing weapon against jellies xD

Biohazard would be best for RJ cuz it poisons + shadow damage. Piercing wise, assuming you want a gun, you can also try the callahan (NOT the iron slug) as another piercing gun option. I had a thread about Blitz usage, and people said that if you have a high CTR, you can spam its charge very effectively on wolvers. If you can repeatedly spam the blitz charge, i think it will overwhelm the poison+ plague charge, just cuz the plague only has a chance to poison, so the damage isnt guaranteed.

Plague Needle has a chance to cause poison, meaning that it only happens occasionally. You are not that likely to get poison status all the time.
Blitz Needle, in my opinion, should be more helpful because of the massive damage it can deal out to enemies, especially Vanaduke.