reference at many levels
I find that it's easier to get "used to" the mix at louder volumes- by that I mean things that aren't sitting right, whether its tone or volume, tend to be revealed less at louder listening levels.... almost like the fine details blur. While it's still crucial to get some air moving and bring it up in the 80-90dB range to check low/high freq. balance (as already stated) I catch the most obvious stuff when I "take a break from loud" and turn the monitors down to literally "next-to-off"... it sort of makes me stop and squint my ears. for lck of a better description, I'm forced to try harder to hear what's going on.
I'd say what's most important is that you reference at different levels. loud & quiet as well as between multiple monitors if possible. The more references you can get, the better! I'll add that I'll tend to lean towards the volume that certain genres of music "require"... for instance making the hard rock or hip-hop stuff "hit hard" when loud, since that's how listeners will tend play it. On the other hand, rarely will you find a dude cranking the crap out of jazz through the 2x15" box in his trunk, so I'd tend to lean towards quieter mixing there.