Unlike airline tickets which can carry hefty change fees, car rental reservations can generally be canceled with no penalty as long as you do so 24 hours or more before you're due to pick up the car. So, while you may not know when the absolute 'best' price on a rental will occur, you can make a reservation at the best price you can find now, then keep checking prices right up until a few days before you set out on your trip and if you find a better price cancel your old reservation and make a new one at the lower rate.
You can also check out consolidators such as Europcar and Rentalcars where you can often get 'extra' fees such as additional driver or one way drop-off fees waived, but be sure to read their fine print carefully to make sure that either you will still have the freedom to cancel or that you are perfectly happy with the price you've got.
As for availability of accommodations on a walk-up basis, that will depend on location and season. Very generally, you should have a choice of possibilities most places and most times with the notable exception of around major national parks, especially those with relatively short seasons most notably Yellowstone and Yosemite. If you decide to 'wing it', be sure that you have some method of determining accommodations on the fly, something far better than just driving around looking.
AZBuck