It's not so much distance and angle, but the light levels under which you view the prints. Under low light levels, there's no problem at all. But in bright daylight or bright "inspection" light, it becomes very apparent. In my experience the problem is the worst with crystal archive II, crystal archive supreme (on the thicker base) is already much better and mostly acceptable, Kodak endura is better and perfectly fine in virtually all situations. But what is acceptable is of course very subjective, and with this mottling issue, it very much depends on the image as well. I tend to shoot quite a bit of low key imagery with large pitch black surfaces and then it becomes a big problem really fast. With eg landscape photos without large patches of solid black, it's not much of a concern at all. I think that also explains why some people have an issue with it while others don't even notice any problem.
And like PE pointed out, this problem tends to become an insurmountable barrier with reversal processing.