IMO monitors give you a much more accurate representation of what somthing actually sounds like than headphones...
depends on what you are doing. When I am mixing a SBD and AUD source, I want to hear what it sounds like to the last detail, especially to make sure they line up correctly, and headphones give me that detail better than monitors. However, in the case where I am adding effects, eq'ing out a source, I will use headphones first, then play it over my playback system, which gives me two points of reference....