The baby makes no difference for your hotel reservation, but for purposes of ADRs, it's easier to log on and add the baby to the overall reservation now. Then when you make an ADR, you can click quickly to add them on. You also get an extra magic band for them, which can be very cute. We currently have "Baby Three" on our reservation for December.
On our first trip to WDW, I made a few reservations without including our 8-month old. I learned from this forum that I should have included him. Over time, I was able to find replacement reservations for everything except Ohana brunch. At Ohana, my wife explained at the front, and they were okay with it. It did take us almost 30 minutes to be seated, but I'm not sure whether that was because of the extra 'guest' or not.