It actually depends to a large degree on the nature of the USB receiver chip and the DAC itself. There was a FOTM dac a while back by Pop Pulse that used the 1796 chip for its Toslink and Coaxial inputs, but a much poorer (forgetting now) receiver for the USB. I would never have even thought to upgrade my USB cable at that point, but when I first plugged it in and ran 16/44.1 files out of my computer into it, there was awful jitter, channel imbalance, distortion and noise like you wouldn't believe. When I replaced the stock cable with a WireWorld Ultraviolet, it resolved many of the issues.
Speaking of which, my brother uses a Carat Emerald to run his iTunes library out into the speaker setup I built for him. I recently got one of the NuForce Impulse cables along with a u-dac2, as I needed something ultraportable for taking with me on the road that would function well with my ultra-high sensitivity Livewires TRIPS. My initial experience with the Ultraviolet cable had really turned me on to Wireworld cable, and so I decided I'd run the NuForce head to head against a Wireworld Starlight USB that he's using (granted, a much more expensive cable). The NuForce cable was eminently respectable, but there was not a subtle difference when switching back to the Starlight. Richer, wider soundstage, much better low frequency extension, better detail retrieval. There may well be well implemented USB dac solutions (be they asynchronous or adaptive) that render the benefits of a higher quality cable largely irrelevant, but in this particular case I can assure you it was a huge difference.
