Sorry, haven't been around for a bit. As far as the first come first served goes, it would probably depend on when you're planning on going. If you're going on a weekend I would definitely call ahead and make a reservation otherwise be sure to get to the ranger station early and get in as soon as they open up(I believe they open at 9:00 am). It's easy enough to reserve a site to know that you'll have a hammock friendly site. Also, if you're going to be getting there after the office closes they have a permit drop box for you to pick up your reserved permit.
I myself always call and make a reservation. Fees are the same either way.
The number to call to make reservations is (415) 663-8054 between the hours of 9:00 a.m. and 2:00 p.m., Monday through Friday.
I didn't get to make it out the other weekend because it was raining and my girl had to use the car and although I'm not opposed to riding my motorcycle in the rain it was very windy that weekend.
Also check out this link
Hope this info is helpful and also that I caught you before you went. If not? Well maybe it will help you out in future endeavors.