I have it fixed at -6.0dB with no link to the Master Volume knob. Monitor and Direct Volume fixed to -12.0, again no link to the Master Volume knob. The latter only controls my headphone volume. Nothing else.
Never had any troubles with those settings. Some sound guys prefer a lower level. In that case I configure it down to -12 / -20.
When I'm driving a guitar cab (very rare occasions) then I link the Monitor Output to the Volume Knob. Similar level settings as mentioned above.
Finally: Yes, that setting can impact your sound halipino - it definitely makes sense to check if you are overloading any other inputs with this output level. Try to experiment a bit