I most definitely do not hold a spot for a family that I've interviewed until I hear a for sure yes, and they come and get my paperwork, etc. I will continue to interview for that space.
Unfortunately, people tend to think now-a-days that not giving you an answer is giving you an answer! Grrrr. I have had one family, I think, call to say "thanks, but no thanks".
Usually, though, if they are interested you will hear back right away. Even if it means them calling/emailing to say "we have 2 more interviews, but you are our first choice right now". If you really liked a family and think that they would be a good fit, by all means I would contact them to say "Great to meet you! I feel like we hit it off, and would like to give you first dibs on a spot if you feel the same. Please contact me within (timeframe), and if I don't hear from you, I will assume that you have found another provider. Thanks, looking forward to speaking to you later!"