For those in Terminal One Uber picks up downstairs from the arrivals level--door "Q" on the ground level. I paid about $30 on a Sunfay night at 11pm to go to just south of Hurontario and the QEW in Mississauga.
I actually found going to the parkade for pickup in the past about just as easy as there were always cars available in less than a minute and you never has to worry about trying to find your car when the driver can't get close to the pickup door due to congestion.