I never use Phones to mix... I can't trust it will give me a real stereo image. Remember that the idea behind stereophonics lies on recreating the band's sound as they were in front of you. But when you use phones, a centered bass drum can be located between your ears - right in the middle of your head. If you try to correct this disturbing effect, you may move your sound field's center away and ruin your stereo image.
Also mind that you must actually hear stereo cabinets to check Phase problems. If you just listen trough phones, you can't hear left and right sound sources blending and canceling each other. This can makes wide open stereo sources (as synths or drum's overhead mics) to vanish when hearded trough conventinal stereo systems.
And there is no way to correct this issues after the mixing is done.
Sometime I may use phones to check how my mix works on an other reference (as I always do at some friend's home, at my own home or at car stereo), but would never use one as my main monitor. Remember we don't usually hear the world on phones, so ambience - the way some room behaves to sound and affects it - is a great part of our hearing experience. Decisions about the ammount of reverberation we should use on one instrument can be influenced by the lack of room reverberance during the Mix.
And there is the fatigue issue.
We tend to get tired faster using Phones than Monitors, so this means we loose our hearing frequency response in shorter time (and our ability to do good mixes, too). The disposition is to compensate highs and lows as we gradually loose our capacity to respond to them after working some hours. Hearing fatigue can makes us unbalance every Instrument's tone on a mix and we should avoid spend too much continuous time in a Mix. And this may be a problem since we get tired faster using Phones.