I don't have experience with the MBD, but when it comes to blending dirt, I tend to prefer just slightly under unity for the clean. I think that it has to do with the voicing of the distortion, though.
The reason I have to prefer under unity is that since the distorted signal is adding more signal, having the clean signal at unity while also having the distorted signal at a high enough volume to be heard in the mix tends to result in far too much signal when compared to the bypassed tone. Backing off the cleans just a tiny bit lets me get a volume balance between bypassed, effected, and dirt signal that works in a mix.
I suspect the exception to this would be a situation where the dirty signal is already lacking in mids or lows. Most of my dirt boxes are pretty full range, and so having the sum low end from both the dirty and the clean would be a bit much.