They usually need to be replaced-
We almost always recommend replacement to be conservative. If any of the people involved had injuries, or any of the vehicles had any damage there is usually no question that they should be replaced.
In a few cases, a parent may make their own choice not to replace a seat. Not having been at the scene first hand, I could not give you this advice. On the other hand, if I was in a minor bumper with no damage to people or vehicles, no visible cracks or stress marks to the carseat and especially if the seat was unoccupied, I would probably choose to keep using it.
The best resource for you is to contact the manufacturer of your carseat to find out their recommendation. In some states, insurance companies must replace seats by law. If the other person's company is paying, they are also often more likely to replace a seat.
To consult a local certified technician for personal advice, please visit the resources at
http://www.seatcheck.net . Thank you for visiting!