I think the golden rule in MMOs is the same as it is here on the Dope: Don’t be a jerk.
Everything else is gravy.
For the record, I don’t just go around randomly assisting others all the time since I don’t see any reason to screw up their XPs (I wasn’t sure if it did in LOTRO, but I know it does in WoW and assumed they worked the same). The game is solo-friendly, so it’s rarely needed.
But I try to remember that most people don’t have selfish motives when they do something that may or may not be nice, whether that help was needed or not. If I one-shot your kill, I really am just trying to help you.
(Helped out a L17 the other day who thought she’d try her hand at a L23 elite (not a signature; an elite) that had about three times her hit points. Once. And then left her to her own devices. If she was dumb enough to continue on by herself when it was obvious she wasn’t ready for it, that was her tough luck.)