It's completely impossible to hear phase relationships while wearing headphones. End of story in my book. Speakers are a must - Full range, standard throw, normal dispersion loudspeakers, in a well-tuned room.
Headphones are for reducing bleed while tracking and checking for anomalies while mixing - Clicks, pops, fades, etc. Not for mixing, and definitely not for mastering (except maybe checking fades and zeroing in on anomalies).