IMHO, you're over thinking it. Technically, some money could saved with very careful attention to dimensions, but in reality, the materials are only a tiny fraction of the cost of a house. Much more relevant are things that run up the labor costs, such as numerous corners and jogs, and complicated roof lines. As soon as you depart from a simple rectangle with a single gable roof, the costs climb significantly.

Another factor is that if you get the outside of the house to come out to even numbers, once you subtract the wall thicknesses, the interior dimensions will no longer come out to nice even sizes, so what you saved on the outside, you likely will spend on the inside.

Not sure but I would concern about the span of a standard joist (24'?). Maybe a structure engineer can clarify this.