My personal line-in-the-sand is whether the question could have been asked at any other non-programming site, or answered successfully without any programming content.
If so, it's not suited for this site - there are hundreds of forums on the internet for interview information, and simply saying, "What's your greatest strength - for programmers" is no reason to have it on this site.
But technically there is no "policy" - the site is community moderated, and the "policy" may change from month to month, day to day, and even hour to hour depending on who's on the site at a given time.
If one wants to post an interview question, they'll quickly find that if it's not allowed, it's definitely in a gray area, and will receive a lot of negative and positive attention as people vote to close and open it.
Generally, it's best to stay away from gray areas, and simply post objective and real programming questions, rather than subjective job-search questions.