We used luggage valet in Seattle a couple of years ago. I think they called us as one of the first groups. I remember hearing "only those with no luggage to claim can leave" or something like that. This was because no luggage would be staged in the terminal so only those with valet service or self-debarkation could leave. I guess there is nothing to prevent you from waiting until later. We had a limo lined up and we were at the airport in about 30 or 40 minutes - they started debarkation at 7am and we were sitting in the airport a little after 8am. I'd say if you don't want to leave early, then 9am for the car is fine.