It's digital attenuation either way. Attenuating at the monitor fader or at the main mix fader will do exactly the same thing when listening to music. The difference is if you *record* what you are listening to (which is what Patchmix is designed for after all), lowering the main mix fader would lower the volume of the recording, whereas lowering the monitor will only lower the volume of the speaker/headphone you are listening in with while the recording is done at full volume. But otherwise, I can't see how it would make any difference whatsoever.
Assuming you are listening to audio with a 16-bit dynamic range, you should have some theoretical headroom to lower the volume a bit without losing dynamic range. If you aren't using any DSPs, it shouldn't matter a lick if you do it in foobar (as long as you are outputting at 24 or 32 bit instead of 16), in the wave/ASIO strip of Patchmix, in the main mix, or the monitor.
