In my first response to this, I made the following statement:

Tenchi (from Tenchi Muyo) adds -dono when he speaks to Mihoshi
and to Washu, I think - but she always threatens him and insists on
'-chan'. But he doesn't do this for Aeka, who is definitely nobility
(even though they are related, he doesn't use this from when he meets
her nor -sama either).

Well, having received a few posts on this matter, I willingly admit
that I was wrong here; it's Washu who uses -dono and very
properly when speaking to Juraian nobility.

Still, anyone care to explain Kenshin's wide usage of -dono to
me or to the FFML at large?

Phil - is curious.