Took a little hiatus from writing these blogs, but I'm back now.
I am going to limit my answer to this topical question to civil matters, especially those in my areas of Family Law and Probate Litigation, although the answers apply to all areas of civil law. In all civil matters, a party is subject to testifying, and in many cases must testify to prove their case. In criminal matters, a defendant cannot be compelled to testify against himself, so I will leave it to those criminal law practitioners to advise their clients accordingly.
An attorney is an Officer of the Court, and therefore has an ethical obligation to not perpetrate a fraud on the Court or any other tribunal. If you tell him facts that you state are true, and they hurt your case, you cannot then tell him you are going to lie on the stand. An attorney, upon being informed that his client intends to lie on the stand, becomes ethically obligated to advise his client to only tell the truth. If the client insists on so testifying, the attorney must move to withdraw from the case rather than take part on a fraud on the Court. If the Court denies the attorney's Motion to Withdraw, the attorney may then put the client on the stand and ask, "What happened?" He or she may not actively elicit false testimony with their questioning. In final argument before a judge or jury, an attorney may not argue testimony he or she knows to be false. In short, the judge - and probably the jury - figures out before the trial that you are lying, and discounts the testimony. Factoring in the fact that perjury is a felony, you should quickly see that there is no upside to giving false testimony.
You are discussing intimate matters with your attorney regarding divorce or child custody. There are facts that you believe hurt your case, or you just want a better result than what the law says you are entitled to. Should you hide or misrepresent the unpleasant facts from your attorney?
In a word: "No". Although the attorney is bound to believe what the client tells them at first, the following negative results are likely to occur:
1. Many unpleasant facts are known to the opposing side, and may be documented to the point that they are incontrovertible. By hiding or lying about these facts, you cause your attorney to be unprepared for the truth when it comes out. If he or she has already unknowingly made misrepresentations to the Court based on what the client has told them, then the attorney and client are at a disadvantage at the outset, and the client's credibility is suspect throughout the case.
2. There are problems with every party's case. Cases do not turn on any one fact, but are more of a mosaic of all of the facts. If properly prepared, the attorney can temper the impact of any negative facts, or accentuate the other facts that favor your case. You rob your counsel of this ability if you hide the truth from him and force him to engage in damage control. You may also be raising your legal fees while damaging your case.
3. If you tell the truth nine times, and lie the tenth time, everybody will wonder if the other nine times you were telling the truth. The lack of candor can also adversely affect your attorney/client relationship, as the attorney may not be able to discern when you are telling the truth. If your attorney feels the need to withdraw because of the lack of trust, you will only spend more money in legal fees to bring the new attorney up to speed.
One final note: many people with a tendency to lie think that they can get away with it in the legal setting. Think again. We do this for a living, and do not readily take the other side's word for things, especially when disputed. Attorneys are trained to uncover evidence, and judges have more experience than anybody on earth in determining whether testimony is truthful or not. What sounds plausible to you in your own head will not often survive legal scrutiny.