Usually I wipe the window clean and just shoot. Most scratches or dust will not be focused on because the focus will be on a part of the wing or ground and you will shoot 'past' any imperfections. Make sure to try and get your lens as close to the window as possible, and as straight on the window as possible, that helps reduce the effect of shooting through the multiple layers of the window. I have taken window shots mostly with my Canon 10-22mm and 24-105mm, but many were taken with the cheap 18-55mm. I have never used a filter on window shots. Also, look for an interesting view if you can.