The only time I sometimes use headphones is while mixing, to check panning and effects. Headphones seem to exagerate these. Once I do that I don't touch headphones, and I NEVER use them for checking realitive volumne levels.
Mastering Engineers do use headphones to check the mix, because some people will listen to the final cut on headphones, but they don't use headphones to make adjustments.
They also use computer speakers, stereo speakers, and electric toasters.