With most towncar services(and TTC is no exception), you can pretty much book a reservation any time. Rates don't change so it doesn't matter if you do it 3 years or 3 days in advance. Just make sure you don't forget to do it!
As soon as you have your flight information, I would go to their online reservation, http://www.tiffanytowncar.com/reservationsform.html and submit my request. They'll email you back with your conformation number. It's just one of those little things that you won't have to do later on.
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