We rented a car for 4 days directly at the airport without pre booking. The agent taking care of us was very, very professional and helpful. We received a Skoda Rapid in a very good condition and all the instructions needed. We will definitely return to Sixt when in Sibiu and perhaps in other locations too.
As general advice: if you arrive early in the morning at the airport they might still have cars to rent without pre booking, otherwise not pre booking might be a bit risky. We paid by card but it can be a good idea to have cash with you.