Big, you're thinking on it too deep, brother! Take a breath and realize that the bow is however you make it.
The belly will be as flat or as rounded as you make it be, regardless of what you scrape it with. On an ELB, most of those were Yew, and as I understand it, yew is extremely strong in compression, so the back was left flat and the belly rounded. But not all woods are best if left with a round belly. Some are more efficient if left flat.
If you're going to have a rounded belly, you can scrape with something flat, and just scrape more toward the edges so the belly is left rounded. I don't know what a cabinet scraper is. But if it has a rainbow edge, higher in the middle of the blade, then of course it will scrape in its own shape-curved. Personally I just use a sharp little pocket knife and scrape flat when I want flat, or round when I want round. You can do almost anything with a sharp little pocket knife.
Does this help?
-Matt