I used to think that sometimes deception was good or even necessary, but at this point in my life, I think that even with good intentions, no matter how badly the truth hurts, being deceived is never my preference, and my answer to your question depends on what you mean by "bad."
Is it always evil to try to deceive someone? No, because, like I said, often people have good intentions. They want to protect you from the truth, so they lie or conceal. Like someone else pointed out, sometimes we even like to deceive ourselves, and I agree that it is very much a defense mechanism. (I won't go into it, but I get this from growing up in the midst of boyband mania and now observing the effects of Twilight on fourteen-year-old girls.)
Is deception usually a bad idea? I think yes, and I might change my mind, but that's how I feel at the moment. At some point, there is always a moment of truth, whether you find it out from someone else or you look back and see it for yourself. And in that moment of truth, there is often a sort of regret. Sometimes there's embarassment. Sometimes there's this frustration because it's like, Ahh! Why didn't I know this sooner? I could've reacted proactively! It's not like you can be fooled forever. (Or can you?)
Oh, jeez. I'm rambling. I don't see how any of this will make sense to any of you. But there's my answer for you.