I have always thought that as much as it would be nice to have that extra income to hold a spot, what if the child doesn't work out? What if they start in 5 months and they aren't a good fit. Then the family has paid you $1000 to hold a spot and you end up terminating in 30 days. Too much of a commitment for me I think.
Aside from that I wouldn't hold unless, it was really hard to fill in my area or the family was amazing or I had someone about to leave at the same time. Holding spots is not worth the income loss in my opinion.