I use clear Hoya HD filters on all my lenses with large front elements like the 24-105 f4 and the 70-200 f2.8 II. I also use hoods on these lenses that help keep things away from the filters.
But, I don't use a filter on the old 35 f2.0, which has a relatively small, deeply seated front element. I've also found that a filter on the old 35 will introduce flare from candles or any small light that reflects off the front element and onto the back side of the filter.
The front element on the 40 isn't as deeply seated and protected as the front element on the 35. Since I use it as a grab shot lens, it can be somewhat at risk. Still, the appeal to this lens is it's size and I don't want to add a filter that might invite contact with stuff since I won't be using a hood. My solution is to use the metal ring from an old rubber lens hood that adds just enough protection to deflect the lens from damaging objects when carrying the camera on my shoulder without a lens cap.
But, I'm curious. Do any of you use a filter on your 40mm pancakes?