I think what is most important when choosing to accept a guest or not, is to see if they have been vetted by other people, and if not, to make sure you're comfortable with them based on a conversation.
But in either case, it helps to be more laid back about these things. I've couch surfed a bunch, and have hosted many people from all over the world. There wasn't a single person I met that I didn't feel safe giving a key to. If I had to leave early for work, I just told them to make sure they lock the door, and gave them a place to hide the key. Maybe that's just me though.
That said, there are numerous people I've talked back and forth with and decided not to host.