Depends on the scene - fine lines and complex detail looks better (well, sharper might be the better word) at higher RPPs as does all the Premium effects (like Soft Shadows) and hi-rez image textures. With Bryce textures I can see a difference between 16RPP and 36 but not between 36 and 256. I've never understood Optimization, but I just do one render pass and write down the time for each setting then use the fastest one. It has no effect on the actual render, just may affect the speed.