When should you tell the truth? When should you say the expected answer?

Many times, when people ask a question, they don’t actually want the truth. There is an expected answer they want you to give no matter what the truth is. For instance, whenever someone says “Do you like my new haircut?” the answer should ALWAYS be yes, followed by ooh-ing and ahh-ing over it. But other times, they want the cold unflattering truth. When should you give the expected answer, and when should you give the truth?