One advice I can give you is if you book it online in advance and they don't have your type of car in they'll usually bump it up to the next highest level free of charge. I've done that many a time.
I always take whatever insurance they offer as well. Last time I rented a truck some people called in and complained that I side-swiped every car along this street I never went on. They told me if I didn't have insurance that I would be fucked, in other words of course.