I always make a res for whatever is cheap at my destination as soon as I know my schedule, and then sometimes make another reservation as we get closer to the dates, sometimes a different car class, but at least a week before I cancel whichever I decide I don't need.

On an upcoming rental, have a res for a sedan, and last night I saw a good price on an SUV, so I made the new res, as I wanted to speak with my family about what car we really want (SUV is nice, but lower mileage and less secure luggage storage than a sedan with a trunk).

Overnight I got an email from Hertz about the double booking, giving me until tomorrow morning to cancel one or they would do so automatically. Not unreasonable, but an interesting change. Res is for three weeks from now.