I have the newer 55 and the 75 f2.8 lens. Can't say which one is least but the 75 definately has some. I believe I saw this covered on Photo.net when they had a section on pentax 67 lenses. Check there.
The 45 has very little; just a bit of mustache distortion. One can put the ocean at the upper part of the frame with little worry of badly curved horizon.