Hmm . . . given that a 35mm focal length with a 35mm camera is really a moderate wide angle, I would think that achieving rectiliniarity wouldn't be all that difficult. I've never used a Pentax 35mm lens, but I've used Canon and Nikon, and I've never noticed any appreciable distortion out of them. I've tended to be a outdoor photographer where my subjects have irregular outlines typically, so I've never had problems dealing with distortion. But in those situations where I've used wide angles on subjects with straight lines, what I've mostly noticed was that the lines tended to angle inward when tilting the lens up and outward when tilting the lens down. So, when take a pic of a building, for example, my pic's lines would tend to tilt inward. Fortunately we have software now that can deal with this natural tendency with wide angles, and it is fairly easy to correct for this sort of "distortion." Even very wide angles, such as 24mm and even 17mm, can be successfully processed. For example, the following two photos were taken with a Canon F-1 and a 24mm and 17mm lenses, respectively. The film was Fuji Velvia 50. Both images were cropped in post processing, and their verticals were corrected. Not much difference between the two images, as a result.
Following is a photo I did take with a Pentax, however. And given your mention of the Grand Canyon, it reminded me of a pic I took of the Painted Desert, which is sort of right next door to the Grand Canyon, and which has some amazing views in its own right. I think the film was Kodachrome 64. The camera was a Pentax KX and the lens was a Pentax 28mm. Because there are no real straight lines, I was able to neglect any distortion created by the lens.
