Go check out this article over at Audio Undone about headphone mixing. That age-old question we have all asked or seen asked many a times. “Should I mix with headphones?”  Blurb in 3, 2, 1:

The key difference between listening to music through speakers and headphones has to do with the way our brain localizes sounds. How this happens is based on the findings of Helmut Haas and is implemented through Alan Blumlein’s invention of stereo. It is sufficient to say that a sound from one speaker will completely mask a similar sound from the opposite speaker if the former is approximately 15 dB louder.

The article is an excerpt from Mixing Audio by Roey Izhaki. So if you find the article your thang think about picking up the book from Focal Press, who publishes quite a few books you should/do own.