I find that headphones are actually poorer for mixing stereo signals. Speakers generally aren't pointed straight at each ear when you listen to music...they tend to be in front of you, pointing towards your shoulders. The stereo field is much narrower.
When you wear headphones, the stereo field is a lot wider, and most producers tend to "narrow" their stereo spectrum to make it sound more like a speaker setup. Then they listen to their song through speakers and wonder why it sounds so mono. However, professional headphones have very flat frequency spectrums.
My profs suggest mixing the stereo spectrum with speakers, and mixing the frequency spectrum with headphones.