T.M.
Honestly, I don't think it matters much. I would probably be putting the baby in the middle myself, just so I can reach over and help out if he needs it. What kind of seats are the older ones in? Do their car seats work in the middle seat? Sometimes that seat is only a lap belt and if they are already in boosters, it wouldn't work so well. And, how easy is it to get the older ones in that middle spot? Do they know how to buckle themselves in or unbuckle themselves? If they do, then it isn't a big deal again, if they don't, you have to reach over to buckle them in, where if the baby is still in the carrier car seat, it might be easy to just plop him in that middle spot so you can strap the other boys in easier. I think it is really just a personal decision. Try it all out and see. It isn't permenant, so if you don't like it, you can always move them around after the baby comes.
T.