If you're using meshsmooth or turbosmooth you can decrease the viewport iterations and increase the render iterations. That way you get more polys at render time without having to lug them around in the viewport.
It all depends, right? If you use low poly modeling with edge weights, you can REALLY iterate quickly. But if you're in ZBrush, yeah, basically iteration means "do that piece over again".
Hey! Yes, break down the scene, iterate on a sub section. Once confident with result (possibly get feedback) apply learnings to other sections. Doesn't have to be polished to the max (it's never perfect), the goal is to figure out an efficient, iterative workflow. Ideally this way you don't have iterate much on the scene…