About rental car bookings; Is it okay to reserve at more than one rental car company, but just end up using one?
It is definitely ok to make muliple car rental bookings. Many people do this, (myself included!), especially when booking a car quite far in advance.
Just make sure that you aware of any cancellation or modifcation fees, if you had to provide your credit card number to book and/or you're booking through a prepaid site. The only thing that I would ask of you, is, if you can, cancel the rental car bookings from the car rental companies that you do not use.
Cancelling car rental reservations that you know you will not pick up, allows the car rental companies to manage their fleets better. If a car rental company has most of their cars reserved, the price goes up on the ones that are remaining, and if all the cars are reserved, people may be turned down for rentals because the companies are holding cars for people who never show up. It's not something that you have to do, but cancelling, if you can, makes it easier for the rental companies and easier on your fellow travellers.