To be honest, I think that a lot of WFP guys are dreaming if the believe that they are leaving the sills clean every time. How many times have we scrubbed bird muck off downstairs sills, we don't even know if it's there when we do tops.
I always did the sills when I was trad, and I run the brush over them now with WFP. I can't say that the top sills are clean though, I don't go up and check them, so I just hope.