Although I've always had joint accounts with my husband, I would never do so with someone I was "just" living with--but I'd not likely move in with someone without being married. I'd more likely stay over or they stay with me, but each have their own place. I'm not prudish, per se, but i am sort of particular about the idea of married vs not exactly married.
But, no joint accounts with anyone not married. Some people have enough trouble with people they are married to and joint accounts, but I never have.