While last minute car rental specials are advertised, they are not something that you
can really count on. Booking your car rental in advance is the best way to try and get the best car rental price.
That being said, you can do both. Do a lot of comparison shopping, well in advance of your rental dates (a month or more if possible). When you find the best deal (try using Car Rental Discount Codes or Current Online Deals) book it.
Unless the rental has a no show policy or cancellation policy, you can cancel at any time with no penalty. If it is a deal that you must prepay, book it if you really like it and make sure to read all the terms and conditions around modifications or cancellations.
Once you've got a car booked, continue to search regularly to see if better rates become available, and check car rental company websites for "last minute car rental deals" up until the time of your rental. If you find something better, then you can book it, but at least you know that you're covered for a rental car, if nothing comes up! Hope this helps and good luck!!
Tips on Finding Last Minute Specials
PrePaid Car Rental