You should choose the installation method that allows you to get the best fit in your vehicle. The car seat is installed correctly when you can't move it more than an inch in any direction when you grab the car seat at the belt path, and when the car seat is at the proper angle, such as when rear-facing.
Car seats are designed to handle crash forces in specific ways. While we know, thanks to federally mandated crash testing, that baby's car seat will withstand crash forces when installed with the seat belt, or with the LATCH system, we do not know if the same car seat will withstand crash forces when both systems are used at the same time. Putting two installation belts through the same belt path could put stress on the car seat shell from two different angles during a crash, causing breakage. Using two installation belts could also concentrate more crash force on a small area of the car seat.
The rule of thumb to follow with installation is to never use your baby's car seat in a way that was not intended by the manufacturer. When you install the car seat using methods not outlined in the instruction manual, you are, in essence, using your child as the crash test dummy. We cannot be sure of what will happen when car seats are used in ways that have not been crash tested and approved. Read your car seat instructions and your vehicle owner's manual to find out how your car seat can be installed. If you cannot get the car seat installed tightly using either the vehicle seat belt or the LATCH system, find a certified child passenger safety technician or a car seat inspection station by visiting Safe Kids USA.
Heather Corley is a certified Child Passenger Safety Technician.