There's nothing inherently wrong with adjusting any of the bus levels, including the Master bus. In SONAR that bus is still 64bits... you can't really squash the sound.
A rule of thumb, in digital work station mixing, is too avoid adjusting the "Main OUTs" or any of the other hardware outputs as that may result in a loss of resolution in bit depth.
A good reason to never adjust your Master bus level and leave it at "0" is that it seems helpful to maintain consistent gain structure on your output so that you can develop consistencies in how you handle the levels in your tracks and sub buses. This makes it faster and easier to work and get a mix started.
best regards,
mike