I've borrowed from Scott Greenfield to emphasize the importance of clients telling their lawyers the truth:
Most effective defense lawyers start with the assumption that their client is guilty. Not because he necessarily is, but because it allows him to detach himself from the emotion of the case to step back and assess the case, the evidence, the jurisprudence, from a neutral perspective. It matters far less what the defendant has to say about his case then what the prosecution has to say. They put on their evidence, and we must determine what we can do about it.
Sometimes defendants are innocent. Far more often, they are guilty. At least of something. And sometimes they lie to their lawyers about it, and about the facts that surround it. If we accept and adopt those lies, we end up the dumb guy in the room, operating from a position of ignorance. This helps no one, especially our client. Without knowing what we really have to confront, we cannot be effective.
It's difficult for many defendants to appreciate that their lawyer doesn't care whether they are factually guilty or not. Many believe that if they insist on their innocence, we will work harder or care more. Not only is this wrong, but it's dangerous. The only real weapon a defense attorney has is information. If the information is wrong, then he's shooting blanks. Feed your lawyer baloney and that's what he will work with. But at the end of the day, the client will have to live with the outcome.
Even though Scott was writing about the dangers of being a zealot, as a prosecutor or defense lawyer, his advice on telling your lawyer the truth holds true for every situation. In short, if you feed your lawyer garbage, you'll get garbage out and you'll have to live with the outcome. So come clean with your criminal defense attorney. We will not think less of you, rather we will respect your courage and do a better job defending your case.