Hey, good question! That used to be super important in 2.79, but in 2.8 it's handled automatically for worlds.
You could mess with changing the Map Resolution, where higher values give less noise but take more memory and render more slowly, but having it on Auto works great pretty much all the time. I have yet to run into an exception.
So, do I see it right, that Multiple Importance Sampling is always turned on for the background since 2.8 and only the map resolution can be set (either automatically or manually with a user defined value)?