Or how they treat the staff at a dinner. Many other routes vs making some people uncomfortable that you may "require" a dinner with a SO. If they don't have a SO, what, invite a friend to an interview?
Good point. I wouldn't formally require the SO. I'd just phrase the invite to make it clear that my SO and I would like to have dinner. If they have an SO but don't bring them that's another potential signal (could be good or bad).
