Helms deep are, to my knowledge, too few npcs to be considered a grind area. around 10, maybe they call more? I dont know.
Edoras is a few more, 15 to my count.
They are far away from any other site though, and will be passed very quickly compared to kenders or centaurs.
Isengaard is simple. Its a trap. There is no way out once you start killing there, except dying or quitting, so you have to leave all your gear, or get someone to come pick it up.
Pax i'd equivalate with Kurinost. I believe draconians are low-xp mobs though.
Goblin caves are also low-xp and i'd range them around RH-titan level, maybe higher, due to the very large groups they may be in.
Mount nevermind has a decent population of gnomes, that could be a grind(lowlevel). But they becomes ghostly and gives you panic, so after a few you abandon the place as it is no viable.