A long time back I had to do some research on USB cable quality, but this was for USB at 2.0 HS or 480Mbps. Basically we took a large selection of the cables available on the market and used a TDR to measure their impedance, reflections etc. What we found was most cables were in spec, and worked fairly well. Some of the worse cables had multiple impedance discontinuities along their length or were right at the edges of the cable spec.
But really I didn't see too much difference in their performance. However... the SERDES for HS USB, and now USB 3.0 are not trivial to design. The IP is expensive to buy, the lower cost vendors do not necessarily provide the silicon providers with a quality solution.
That means to you that often I have seen devices that the developers have tuned to work after much frustration with the correct length, or just a particular cable. In their lab they know they have seen problems with perhaps very short cables or very long cables for instance (I've seen both). So the simplest thing to do is to put in the manual "Use the cable we recommend", and if you don't they're not liable for any support.
As far as I know there are no "classes" other than maybe this cable is USB 2.0 HS certified or 3.0 certified. Unless you are going to test before or after you buy I would say your best bet would be to stick with a brand you recognize. Really I don't think you'll have much problems unless you're trying to buy no-name cable off E-Bay or something. Even then if you test it you might be surprised at how well it works.