I wouldn't worry about it until you are 76 days from your arrival! Just joking. I too would go ahead and book the high rate. As mentioned previously, the worst that can happen is that the prices stay the same, but likely, the prices should drop as you get closer. I too check regularly and have often re-booked 2-3 times within a week from arriving in Vegas. That being said...... you must book with the rental car company itself and not a third party broker like expedia, hotwire, travelocity...etc. Once they got your money, it can be next to impossible to switch or cancel with the 3rd party companies.