N.,
I have 5 boys and 3 of them are still in car seats. I put the oldest child still in a carseat in the very back, simply because he is capable of buckling himself in, all I need to do is visually check to make sure its done and I can do that from the 2nd seat. I also put him in the middle of the bench seat but the only reason I do that is to seperate my 2 oldest boys!
For the youngest two, I have done it both ways, while they are in the rear facing car seat I put the littlest baby behind the passenger seat, because if they are fussing while we are driving I can see him and if he loses the paci I can reach around with a fresh one (I keep several extras with me in the van within easy reach) while we are at a stop light, or if necessary I can pull over and tend to him without actually having to get out of the van.
When the baby graduates to a forward facing seat I move him directly behind the drivers side. This makes loading kids much more convenient. You can get them all in from one side of the van. The second oldest can climb in and pull the seatbelt over for you (if they still can't buckle in by themselves) and you can click it without having to go around to the other side of the van. You can then buckle in the youngest, shut the door and hop in and go.
At this point we kind of have it down to a science! LOL Congratulations on the new baby and good luck to you!