You should always be upfront and truthful with your superiors. First, your boss doesn't like surprises, they always want to know whats going on. Second, being dishonest will cause the boss to lose trust in you and most likely lose your job, or put you in a situation where it is much easier to lose your job.
Third, if your boss knows they can trust you they will rely on you more, causing you to be promoted sooner.
