I was on my mobile phone in bed last night when I typed my post, so I'd like to clarify more.
On my 17mm - No filter. Never really thought I needed it. Since the lens is fairly compact, I don't worry about damaging it too much. I do have ND filter step ring for it though, so I can use my 58mm ND filters when I need to.
On my 14-42 Kit lens - B+W MRC clear filter. I think I paid 40ish for it. Was running as much as the high end Hoyas. I only use it when I hike in dusty areas or at the beach. Windy areas with lots of dust and sand, usually. For everyday use, I don't keep it filtered. Because of the lens design, I worry more about internal issues than I do breaking the glass on the front.
On my 40-150 4/3 - ND filters only. I don't have a clear filter for it. I only use this lens on occasion, so I find no need for one.