I have mixed feelings on the pre approval. As a host Im happy to respond and pre approve, but I noticed a trend that pre approval led to less bookings. I think I know why, as a guest if you are pre approved air bnb seems to hound you to book, I think I got three reminders in 24 hours to book. It puts a lot of pressure on the potential guest. I know in my situation, I was trying to organize a group which takes time, that pre approval was just another annoyance. So now as a host, I respond, and let them know that they are welcome to book at their convenience if its still available.