i've been experimenting with "optimize last render pass" since many other ppl have recommended it. mostly with relatively uncomplicated things. a few of them have come out with smudgier areas, or a little flat. render time was 1-3 hours using a spectral atmosphere, for what was previously 7 (not counting godrays). so i will probably be using full quality most of the time. which means where do i put the quality boosts, these increase the render time. for example, i'm often running 2+ on the sky, fog & haze panel. lowering this unfortunately changes the color/quality of the sky too much for my liking. seems it's easiest to decide first what you want the quality to be, THEN design the atmosphere for best results. some ppl have said you must have 4+ for rendering godrays, that's the only one i've ever tried. most of the time i've aborted the render, since it wasn't practical & kept my computer tied up too long. if they've fixed the memory leak it's now worth a try to render godrays.
 
notice they also fixed the anti-aliasing, where it got stuck on "soft" & water turned to metallic sludge. crisp seems to work now.