This is terrible advice if you are booking for a busy rental time. Rental agencies often run of cars. Make a reservation at the best price you can find, then check it occasionally to see if prices drop. Prices do NOT always drop at the last minute either.
I am relating my own experiences. I have rented cars at MCO close to 20 times. At all different times of the year. I always use Hertz. They have never come close to running out of cars and I have ALWAYS scored at least a 30% savings over my earlier reservation (which I said, I always make a backup reservation just in case). One time, my reservation was $30/day for an intermediate car. I literally rebooked during my layover on the way to MCO. The new price was $12/day! That was my cheapest rental ever. It was a Fri-Mon rental during WDW Marathon weekend in January 2015.
This method is basically foolproof with Hertz rentals out of larger international airports.