DS is 8 and he's talking about a sleepover at a friend's. I know his mum but not particularly well. DD is 6 and very confident, so I'd be happy to let her stay with a parent SHE knew well, regardless of how well I know them, as to me that's the more important thing.
Even if it were my best friend, if I didn't think DD would be happy there I wouldn't send her, but she knows other mums better than I do from playing at their house and to me, if I trust them enough to let her go for a play & for dinner I don't see a difference in going to sleep there.
My ds1 is 8 and I'm not keen on the idea of sleepovers yet although I know a couple of his friends have started to have them. At this age I'd only consider it if I knew the parents well- he's quite shy & I don't think he would be comfortable just yet sleeping away from home. If the situation comes up & he really wanted to go I would consider it but as I mentioned before not if I didn't know the family reasonably well.
My DD started the occasional sleepover at 7 but only with families I knew well. I would not be happy with my DC going to households I didn't know reasonably well at primary school age. I would be concerned that they would be unhappy about something or want to come home but not feel they knew the friend's parents well enough to speak to them.