As the DAC accepts 32-bit inputs, it really doesn't seem like adjusting volume inside Media Center would be harmful anyway. That still gives you 48dB of attenuation with 24-bit files, or 96dB of attenuation with 16-bit files before they start to be degraded.
It's more important with a DAC like mine where it only accepts a 24-bit input, so as soon as you start reducing the volume in Media Center with a 24-bit file, you're discarding information.
The DAC itself has a 48-bit internal volume control, but without the ability for Media Center to control that, you can only take advantage of it if you adjust the volume manually rather than using MC's Volume Leveling features.
That said, the DAC measures about 21-bit on its analog outputs though, so you get about 18dB of attenuation before anything would even theoretically be audible, and in reality, you're not going to hear anything down at -126dB anyway. Volume leveling is usually less than 18dB of attenuation as well.