I think that if your working distance and brightness of your lamp at the working aperture is adequate to judge focus either on the paper or through the grain finder/magnifier, you might as well focus at that aperture and leave it there. It will save you a step of having to open all the way up and then stopping down to working. If you cant or are not sure of the focus, then do it wide open, or at whatever aperture allows you to see the image clearly, and then stop down.