Well there are pros and cons to mixing on headphones, same too with using monitors. Ideally speaking, using both is better. Or failing that, really knowing what sound you're going to get with what you're using and how to compensate for it.

I find panning and stereo imaging more difficult to gauge on headphones. Listening on cans is different to listening to speakers. Each ear only hears whats coming from the relevant channel, but with speakers both ears are hearing signal coming from both left and right channels. There are some timing differences here and it forms the basis of the stereo illusion (phantom image between speakers etc). With headphones you don't get this. Personally I never feel it to be a true stereo image.

But that's not to say it doesn't have it's uses for reference. Also if your room acoustics aren't great then headphones can provide a good option. Really though, it's about getting to know the response of the headphones and accommodating for it.

Cheers,
Deryk