A tail heavy airplane will generally be overly sensitive to elevator input. (a little move of the stick causes a LOT of response from the model.)
So just because you had to trim it down doesn't nesesarily mean it's tail heavy.
Basically if it flew ok after the down trim then you're ok in the balance department. just rotate the control horn on the servo a bit so that it has the same amount of down trim you needed when the trim slider is centered, and you should be all set.
Somebody feel free correct me if I'm wrong here...