We've always tried to preboard, both child and car seat. Depending on the model of seat you have, it can be tricky to buckle it, especially in a crowd. We learned that the easiest way to fastern ours in was from behind - standing in the row behind the seat, and reaching through to get the strap tightened - an FA on AS showed us the trick - but it is hard to do if someone is already in that seat behind you. Also, my son needs a moment or two to look around a new place - he settles down faster if he can check out the plane before we strap in him. If he can preboard and look around while I fasten down the seat, the trip goes easier for us. The one time we did the staggered boarding, Mom and seat first, Dad and child last, he got on board the plane hyped up and didn't take well to immediately being strapped down.
Your kid may vary.
