It depends on the DAC. Many DACs -do- need the 5V while some do not.
Some DACs -require- the 5V Vcc over USB continuously or momentarily while others don't need it at all. That's one of the reasons for experimenting by blocking the 5V line in the USB-A connector. If the DAC stops working after blocking the 5V then you have just determined that your DAC -requires- the 5V over USB to function. If that is the case you can then break the 5V line and inject your own 'clean' 5V to see if it helps SQ. YMMV.
In my experience I've owned DACs that required the 5V over USB and others that did not. There was no correlation to suggest improved SQ from DACs that did not require the 5V. In fact, I had an Ayre DAC that did not require the 5V over USB at all but my PS Audio DACs does but sounds ever so slightly more to my liking so I kept it. Sold the Ayre long ago.
