I'm not sure how clean your customers sills are, but mine are always a mess, especially the ones under patio doors.
I do clean them, but the brush gets terribly dirty, and even if I first clean the frame on the next window, the dirt still gets transferred to the window. I'm fed up with rubbing my hand through the brush to get rid of all the mess. Makes rinsing on near impossible with the brush being contaminated and all.