I undertake to observe the rule to abstain from false speech. That is the Buddhist version of Thou shalt not bear false witness.
I try to be honest but total honesty is ultimately anti-social. "No that dress doesn't make your butt look fat, your fat butt makes your butt look fat." Plus, it could be dangerous.
However, I do lie every time I donate blood. I blatantly tell them on their form that I have never had sex with another man. To date I have donated over 5 gallons of disease-free blood, which would come to an immediate and permanent end if I once told the truth on their form.
So, honesty is not always the best policy if a stupid, discriminatory question is asked.