For us, book as soon as you know you want it. We've had people book several months in advance and we make sure to honor them (the "worst" that happened is that somebody got the whole house to themselves for the last few days of their stay while we were in France).

I would just ask what the check in procedure is. If they are on top if it then they will have one. Also, again, look at the reviews. If someone has had a dozen guests who liked it enough to review then you know that they are serious about it, and canceling on a guest would completely ruin the reputation of someone like that.

We would only cancel on a guest if we physically could not host them (room was on fire, etc).