Depends on the DAC, they are not all created equal. Say you have a Cyrus 8 DAC integrated amp, its DAC has USB and Optical (Toslink). The USB accepts only 16bit / 48kHz whereas the Optical accepts 24bit / 192kHz. So if you want hi-res music on that DAC you have to use Optical.
If you have both, give them both a listen. Chances are you may not notice a difference.