From my experience, stations are assigned in order of join date. During the pre flight briefing, they would go down the list and the most senior (i.e. the attendant that has been with the airline longest) gets to take his/her pic. It usually goes 1A, 1B...etc, but one actually chose Y when her name was called, although there were still stations in the back of J available. Told me she loves F, but chooses Y over J on this flight full of OP-UPs...