I usually choose within 15 mins of when I land. If there is no price change and my options, I pick as close to when I land as possible. Once I picked about half an hour after we landed. We got there about 15 mins early and had to sit around and wait for our rental car time. Now I would rather be a little late to pick it up then have to sit and wait on it.
I'd shoot for 9 am. You can be off the ship as early as 7:30, or so. Once off the ship you need to find your luggage, clear customs and catch the shuttle to the car rental office.