I use multicoated filters, such as Hoya HMC super multicoated UV(0) filters on all my lenses with 77 mm filter threads and smaller. On the large telephoto lenses (e.g. 500 f/.4), the lens hood does an impressive job of keeping the front element clean. I find that for the lenses I do have a UV(0) filter on, the filter gest dirty and grungy often for the imaging I do (from rain to spray from waterfalls, ocean salt spray, dusty environments, etc). The combination of water and dust can be permanently destructive by cementing dust to the optical surface (a reaction of the coating, the dust, and water). So if you work in difficult conditions, I feel it is best to use a filter, and clean it and change it often. I buy new filters every year or so to keep performance in top shape.