I wonder if there is any penalty to having SSS, IDL, and displacements enabled vs. disabled when nothing is using them?  I'd assume not.  I leave most everything on by default, as I tend to be careful having that stuff the way I want it for each prop and figure in my library.  I generally prefer quick over quality, but start with high quality settings and then turn the settings down as the scene becomes more complex if the render time starts going up.