I could see her pulling off calling him either 'Owner' or 'Sir' all the time, and he would be like X3 'Call me Shunsui! Or at LEAST Kyourauk-san, Nanaooo~chaaaan~!'

I think everyone pretty much will go by last names except those who never do, like Ikkaku and Yumichika... Hitsugaya will still be as disgruntled as always and only Ichigo and occassionally Matsumoto will call him by her first name.