I use a Sigma UV filter on mine, just like I did with my previous DP2M. The front lens element is quite exposed on the DP2, so I prefer to protect it a bit.
Adding the UV filter increases the risk of flare (the only real life image degradation), so you want to use a hood. Ironically, adding the hood reduces the need of the filter in the first place ;-)
With my DP3M I used neither a filter nor a hood since the front element was recessed at typical focus distances.