I've learned not to say I don't know. Every single time, I get told, "that's gawd. Mmhm. He takes keer of everthin."

I taught my students it was proper to say, "I don't know and I'll be sure to check." If I didn't know, I would ALWAYS look and come back and inform them - few really cared, but it proved my point.

I've learned a while ago that you are not required to attend every argument to which you are invited. As for direct questions, if I don't know, I say I don't know. If it's important to me, I'll make a note and follow up on it. Except for little white lies (no, no, those jeans look great on you), I don't give answers I know are not true.

I never admit I don't know....its always "I'll have to get back to you on that, .and I only lie when in dreadful fear for my life. .

Zero!

I tend to fiercely protect my own credibility, and I will always answer honestly with "I don't know" than lie to pretend I do. There is no shame in not being a know-it-all, and no shame in saying "I was wrong about that." It's far more important that people understand my sincerity.

“I don’t know anything. Ever. It’s really quite relaxing.”

I don't know.

I don't know a lot of things and try not to lie about it.

Let me modify the question you are giving and change it to a simple "I don't know." I say it a lot in religious discussions without going on forever about bible doctrine. One of my favorites is "I don't know and you don't either. You're making it up." If this gets back to "the bible says" I come back with "why should we believe the bible?" As for the lying part, I usually lie by omission. Keep my mouth shut and don't go far enough to be a part of the lie.
I submit this as an atheist. If you like you can call me an agnostic atheist. I really do not know and I am not making it up.

I always start with. "I don't know." as the default, because in most cases I think that that is the most honest and accurate answer, and that it is that basic level of, honesty, which makes most of the sceptical ideologies better than theist ones.

I probably don't lie that often these days, since now that I am retired and post self employed, I chose not to mix with people I would feel the need to lie to, any longer.

Well speaking on a personal basis here, when it comes down to discussions/debates regarding religion and religious doctrines, etc, I can honestly state that I never need to say the words "I don't know" since I have made it my aim in life to discover everything possible regarding Christianity, its tenets and doctrines, its ACTUAL history, the bible and its TRUE origin, etc, etc.
And if a subject other than religion comes up and I'm unsure of the answer to a question, etc, THEN I make it a point to find out whatever it is that I need to know and to understand as well.

I say 'I don't know' all the time as an agnostic atheist, but I don't know how related to my beliefs about religion it is. I think it has more to do with my desire to not be wrong. If I don't know something, I either say I don't know when asked directly, or simply keep my mouth shut if I wasn't (no point in volunteering that you don't have anything to add).
As for how often I lie, I imagine pretty often, but rarely, if ever, in an attempt to be maliciously deceptive. Sam Harris makes a really good case for never lying about anything ever, but I just can't imagine it in practice. Unless you were famous for not lying, thereby providing anyone you come across foreknowledge that you wouldn't lie to them at all, the self-imposed inability to lie would definitely come off as a personality disorder or maybe autism.