I always keep UV filters on my 35mm lenses, but strictly for protecting the front element. Since I use MF and LF for landscape and table-top only, I don't worry nearly so much about the front elements on those lenses being damaged so I don't keep the filters on all of the time.
With the 35, I'm likely to be shooting in a crowd, from a moving vehicle, or countless other situations where the front element of the lens might be endangered. If I find myself somewhere where the front element of any lens might be damaged (for example, on a windy day with a lot of "stuff" blowing around) then I'll put a UV filter on the front of any lens I bring out.
I doubt that my eyes could detect the image degradation between shooting with a high quality UV filter and without one, but I figure why take the chance unless there's a good reason for it.
I'm thinking you'll get as many opinions on this as you get responses. Goodness knows I've questioned myself for keeping an extra layer of glass on the cameras with the least area of film (and therefore the highest chance of showing any degradation from the filter). The reality is that I use the filters this way because it's what I'm comfortable with, and I have no better reason to offer.