It completely depends on the situation... the only absolute rule is that you SHOULDN'T make any tonal or timbral decisions only using headphones.
ALL you have to do is try it once and you will see - that ripping guitar sound you chose while listening to headphones that sounded AWESOME in the cans, will not be pretty 99% of the time on the monitors.
Once you've made your sonic choices, use whatever is most convenient for monitoring.