I booked my Hertz car rental using Marriott rewards points, from the initial read it looked like I could cancel anytime no strings attached, but had to pay 35 dollar for cancellation. Per me, better not to use Marriott rewards for car and flight (unless you don't have a choice, like I did).
Thanks for the warning. I've been curious about using points for booking flights and car rentals. I came close to booking a flight once with points and during the checkout process I realized that I wasn't able to pick my seat so I decided against completing the transaction.