Buffer underruns do cause quality reduction. There are small gaps between the burned sections of the disc because no burner can perfectly resume the burn. I've noticed that one of my fussier CD players, for example, chokes on a disc that had buffer underruns (lots of skipping) whereas a clean, non-underrun burn plays cleanly. Some of the newer buffer underrun technology (like Seamless Link) works better and the gaps are smaller but they still exist.
It's always better to be able to burn a disc cleanly if you can. Avoid multitasking when you are burning. If you really need to multitask, burn at a slow speed, like 2.4x or 4x (8 or 16x for CD).
You say that your DVD burner is on a USB cable so DMA doesn't matter, but where is your data coming from? If your local hard disk, is DMA enabled on your hard disk and its IDE channel? You have to not only be able to push the data to the DVD burner fast enough, you have to get it just as fast too. If you're burning off a network device, you are going to get lower throughput than if you burn off a local hard disk.