Yes they only charge you if they catch it for you, things is it's a matter of balancing, backorder low quality domains and it will cost you a fortune are they are likely to get caught but order only quality domains and it might not cost you much but the chances are you will never get any caught due to too much competition.
Ask yourself this > Is likely I could reg the domain fresh once it's dropped ? if yes then don't book it, if no then ask yourself is it likely lots of others will try to catch it ? if yes then you're probably wasting your time however it doesn't hurt to try.
Go for meduim level domains, .org.uks might give you a better chance of a decent generic.
Good luck