This issue has been discussed on this board previously and the discussion has gotten rather heated. Some of the folks expressing to be such sticklers on this subject are also sticklers and negative on many other subjects, and if you are new and not familiar with thier ongoing "theme" you may give them more credibility than is probably due. This discussion has had over 40 posts when previously discussed.
I think you stated very well in your original post how having one more child may force you in to a two bedroom that you really do not need. A one bedroom may accomodate your family just fine. I agree with that position. Three kids in one room and the parents in another is a very comfortable vacation situation for us.
Growing up, as kids, we always just had sleeping bags on the floor and really liked it. I certainly dont' have any problem with 5 people in a one bedroom, and Disney guides have expressed that an extra person is OK. They just won't be providing bedding and towels to cover for them. I think that's fine, we can bring our own towels, etc.
The discussion has gone all over the place, including wear on the rooms, resort capacity, etc. but there are local people not staying at the resorts crashing the pools that aren't really being delt with, and all kinds of issues that could be delt with prior to restricting families that may decide to have a third child.
Talk to several guides. You'll find that 5 to a room is OK. Also, BWV does have studios with an extra day bed that are rated at 5 capacity.