I'm kind of late to the thread, but I wanted to say that if you want to limit your child's sleepovers (appropriate, I think) you should also be prepared to put yourselves out there. Be the house for other kids to come to. Get to know the friend's families. Do things so that not only do you feel comfortable, but that you aren't squashing your child's social life unnecessarily because no adult picked up the phone.
We are probably more lenient than you, but we will pick up the phone even now to confirm that yes, an adult will be home, yes, so and so's mom is driving, etc. If there was a party a child wanted to attend but we didn't know the family well, we might say go to the party but we'll get you at x hour instead of sleeping over. Or we might say, "Okay, but only if I talk to their parents first". In middle school it is VERY common for kids to hand out invites and never get the parents involved in the actual inviting. If my child wanted to invite someone, I'd talk to the parents as part of the RSVP, but not necessarily call up every kid's house before sending out the invite. Part of the party planning would be to trust that my child was inviting his/her friends (who I might have seen at school or heard about prior to the invite).
Obviously you wouldn't deliberately put your kid in harm's way, but I think parents need to balance all factors. Would you disallow your child a band competition trip because you don't personally know all the chaperones? Or will you decide to BE a chaperone to allow your child the opportunity to attend?
I was also taught that even if my parents knew the family, if there was ever any question or need to come home, I could. At any time. I think it's also important to allow your child to make judgment calls and support them. Could be as simple as not liking their dog, but that's still legit.
First impressions may be the right impression, but sometimes you have to look twice to really determine what kind of people they are.