He/She

I still find it difficult to figure out what the conventions are for 'he'/'she' pronoun choice in philosophical examples. Back in the mid-90s when I was an undergrad in Copenhagen my teachers told me to use '(s)he'. But I was quickly corrected by my supervisor when I started graduate school. He told me "just use one or the other". For a long time I wondered whether the 'or' was supposed to be exclusive. I concluded that that was probably what he meant. But he didn't follow his own advice. He would tend to make the farmers, truckdrivers, prisoners and serial killers females, and the nurses, homemakers, and wedding planners males. Since then there has been even more confusion. Some have told me to alternate for reasons of fairness. Others have told me to avoid 'he' and 'she' altogether and only use 'one', 'I', and 'you'. Yet others have told me that women can choose freely but that men must use 'she'. So which is it?