When we address someone as "Prabhuji" it means we are accepting that person as our master. Since every devotee (vaishnav or vaishnavi) should be accepted as master the ettiqutte says we should address every one as Prabhuji.
However, there is nothing wrong to use the word "mataji". It's usage suggests that we are accepting the person addressed as our mother. This is in fact the rigt way to look upon any woman.
What is of prime importance is the respect and gratitude we have in our hearts for the devotees. As a litmus test everytime when we address someone as prabhuji or mataji we should look into our hearts to see are we really accepting that person as our master. Given oppurtinity can i serve that person to the best of my ability? This will also ensure that our words are coherent with our thoughts.
Looking foward to hearing from the experts.
Your Servant (bless me so that i may realise that you are my master)