It depends a lot on what you mean by distortion. An optically-perfect wide angle lens shows perspective distortion when the viewing angle is less than the taking angle - for example the taking angle might have been 80 degrees, but when the picture is viewed it might only fill 40 degrees of our vision. That change in perspective is often called 'wide angle distortion' and it is nothing to do with the quality of the lens. If you viewed the picture close enough so that it occupies 80 degrees of your vision then it would not look distorted.
There may also be distortion arising from optical imperfections - commonly slight barrel distortion in wide angle lenses. That is usually correctable in post processing.
On full-frame 35 mm I find that anything wider than about 35 mm to 40 mm starts to have a distinct wide-angle feel to it. The wider the lens angle, the more of an immersive feel you can get. I think that starts at around 24 mm on full-frame. Though I would not call it distortion, I'd say that anything from 24 mm (16 mm on a D200) and wider had a distinct wide angle feel, 28 mm to 35 mm (about 18 mm to 24 mm on a D200) was moderately wide but not excessively, and 40 mm to 55 mm (roughly 26 mm to 36 mm on a D200) was normal in terms of perspective.
Best,
Helen
