I pick stalls based on what looks clean to me. If the first one I'm comfortable using is next to one already in use, so be it - I don't care if there's other empty stalls that might be clean also. There's no reason why I shouldn't be able to use that one. Plus, there's always the fear that the stall may not actually have TP, in which case having a person nearby can come in handy. I haven't had to ask yet, but I've helped others out in the past, so it does happen.