I book rooms first simply because they are changable or refundable where as airfare isn't. This way, if my plans change, it is easier to adjust my hotel reservation. Also, in the past, I have found great airfare but no hotel availability. I would hate to spend all that money on airfare and not have a place to stay.
For my current trip, I found it is now much cheaper to fly in and out the day before I originally planned. All I have to do is adjust my hotel reservation and I've saved a bunch of money. Had I booked airfare first, I wouldn't be able to adjust my trip.