Quick question...
Do full term, healthy, average size newborns need the extra head support things that go into car seats? I have a few friends who have had babies and used them, but they all were born at 36 weeks or earlier, so I'm not sure...do I need one for my full term baby?
Re: Car Seat Inserts
We'll miss you sweet Debbie Girl (4.21.12) and sweet Cindy Girl (8.9.12)

This precisely. My DS came home from the NICU at almost 7 lbs, which is pretty close to average for a newborn, and we used them. Just watch how your baby sits with it to make sure it's not too small. His carseat came with two and we only used the larger one. But yeah, don't buy any extra ones or install ones you got as a gift. The carseat meets safety regulations - the extra pads you can buy separately do not.
This. Read the manual and it will tell you until how many pounds they need to keep it in there.