At MCO they will assign you a vehicle at the counter or, if your a Hertz Gold member, you are assigned a vehicle but pick up the keys from the booth in the parking garage.
We had a FS car booked and were assigned a Chev Equinox in 2015 but we wanted a car. I asked at the booth and they gave me the choice of 3 other vehicles (Altima, Impala and Fusion). Took the Impala, it was an easy change and just had to ask.
We didn't get to see the vehicles first, were given the keys and the space number it was parked in.