Whether or not the question is stupid I can't say but I can say it is not an easy question.
Each rental company has its own rule on showing up late for the car. THe company is more likely to hold ou to its rule if it is a busy time for car rentals, such as Daytona Week. Once a rental company would not give me a car early unless I upgraded and paid more, although they did give me the car I asked for at the apointed hour.
If you are concerned about timing your rental pickup to match your arrival, you might consider a rental company that lets you specify your flight number. Then they have the car ready when your flight arrives.
You can also send one family member to get the car while the others are waiting for the baggage at the carousel. Whether you wnat the family to take the rental shuttle to meet up or instead the driver return to the airport to meet the family is up to you. But do realize that many pylons and other curbside features look alike and this makes it difficult to pinpoint a rendezvous point at the airport.
It is not a good idea to use the airport garage if all you are doing is taking advantage of a free half hour of parking. You won't make it in and out within 30 minutes.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
It is not unusual to, when arriving at the airport to pick up someone, to find the entire curb lined with cars, engines off, drivers waiting with arms resting on the windowsills. When you pull alongside to pick up your party, ask the policeman to shoo one of them away so you can pull in to the curb.
