In what cases is it appropriate to refer to unknown gender as she?
While technically it should refer back to a plural, the singular 'they' is commonly used and, indeed, has a long history in English usage (back to the 16th Century).
You can use the plural pronouns ‘they’, ‘them’, ‘their’ etc., despite the fact that, technically, they are referring back to a singular noun:1
e.g. "Whoever it was, they should come forward."
Only in certain situations such as extremely formal writing would I not use a singular 'they.' This is due to the fact that it still does irk some people.