Most car rental companies rent out carseats. If you do, make sure when you call you find out the model number of the seat YOU will be getting so you can check for recalls. Also get the EXACT expiration date off the seat. It will either be on the shell of the seat or on a sticker on the side. Also ask what they use to wash the seats (specifically teh straps) and how many times the seats are rented out before they are destroyed.
Studies have shown that many rental carseats are either recalled or damaged. Some have hairline fractures. I have heard horror stories on the car seat tech board about people renting seats and then the places not having any when they get there, dirty gross old expired seats.
You never know of the company washes the straps in detergent or just wipes them with a small amount of water which is what ou should do.
If it were me I would either bring my own seat (gate checked to avoid damage or loss) or have whoever is driving the car buy a cheap one.