Winning Trial Advocacy Tips has a great post on the subject of whether you should ever call a witness a liar.
An excerpt:
Because we’re lawyers, we don’t have any problems believing that someone will take the stand and lie to us. But jurors don’t think like that. Maybe they’re more optimistic than we are, or maybe they don’t get lied to as often as we do, but most jurors I’ve met prefer to think that any witness who takes the stand is going to be honest with them. (Yes, they even expect 10x convicted felons to tell the truth.) If you attack a witness’s testimony by calling him a liar, you’re going to need toprove that he lied. If you can’t prove that he lied, you face an uphill battle trying to get the jury to disbelieve his testimony.
I encourage you to read the entire post.