I don't recall Akane ever calling Ranma "anata" either, though she not
infrequently uses the coarser "anta".  However, I would remind you that,
when they pretended to have actually married for Ukyou's benefit in that
Ukyou's sauce storyline (tankouban 19), Akane *did* call Ranma "darling"
(and he referred to her as "honey").  

I've heard the anata several times.  She does use it once or twice
in the Ukyou sauce storyline, and somewhere else that I can't remember
at the moment.

For those who are interested, Nabiki does it a few times as well :)