Yes book ahead, Mrs F always book Cabrera Medina or Auto Reisen on t'internet. They're generally the cheapest and both have desks at the airport.
Quick tip, if there's not much in it cost wise the queue is usualy shorter at Cabrera Medina and even if it's longer it's quicker, the autoreisen system is like taking a mortgage out. Also when you drop the car back you just drop the keys and leg it at CM, AR check it and prat about for ages.
I guess you'll find hiring for nine and driving yourself to and from the airport will be cheaper than hiring for five and paying for airport taxis. Plus gives you the flexibility

Thats what we do.