I have seen some back and forth among photographers on filter use. Some say (generally EMPHATICALLY) not to use filters because (1) you're putting inferior glass in front of your expensive precision lens, robbing it of performance; and (2) in the event of an impact the damage to the lens will be worse if a filter is fitted. Anecdotally, I have tried things both ways and come away with some impressions. First, unless you're using horrible quality filters, the image quality will not suffer EXCEPT in unusual situations. The UV filter does add an element that can produce flare--this is why people spring for super-extra-digital-multicoat filters that cost $100 (and because some folks have to spend money excessively to convince themselves they're taking good photos). Second, the front of the lens becomes easier to clean with a UV filter fitted. You now have a flat surface instead of a curved element, and a LensPen DigiKlear will clean it superbly. Also, if it gets some solvent on it--say, toddler drool, mud or just plain rain--you'd rather it be on the filter. That can be replaced relatively cheaply. It's possible that the lens would fare worse in a catastrophic impact with the filter on, but how often does that happen? More likely you're talking about scratches. Again, better to damage the filter. All that being said, I don't like to use filters on lenses that use external focusing, e.g. Canon 50mm f/1.4 USM or Panasonic Lumix G 20mm f/1.7, because you're adding mass that the focusing mechanism has to move. Those lenses are already a bit slow to focus without any assistance from extra mass.