This is a topic I’ve thought quite a lot about, so I’m going to go and try and attempt and put down what I think here.
Briefly put, I’m going to be trying to argue for an absolutist position of that we should always tell the truth - or that at least to take this as one’s personal policy is a coherent idea. A lot of this comes down to instinct and my own persoanl personal prejudice, so whether I’ll be succesful or not is another very different matter, but anyway… 
First thing is to clarify what I mean when I say that you should always tell the truth. I think it’s probably better and more helpful to think of it as promising never to directly and intently lie. That is, you’re allowed to joke, to exaggerate for effect, to not always be right because you might be mistaken. You are also under no obligation to volunteer full information - being tactful and keeping your mouth shut is obviously a very useful option… To lie is to state what you know is false with deliberate intent to decieve. Both of those conditions are needed. I even think that you should be allowed to deliberately deceive people with the truth, as long as you do it by taking advantage of their preconceptions or by withholding information rather than direct falsehood.
I realise that that’s a very different line to draw exactly, and some people might ask what exactly the fundamental difference is. I confess that again it at least partially it comes down to an instinctive feeling that there’s difference between ‘not being stupid’ and trying to get the better of someone, and making use of the lie direct. Trying to justify this, I think it comes down to a trust issue, that at the very core of it if you want the truth for someone, you can always ask them a simple direct question. If I want the truth, I should be able to get either it or simple silence.
I think it’ll probably next be helpful if I split the cases where it is often argued that one should lie into two areas: firstly, the trivial “yes, honey, those trousers look great on you” area, and secondly, the hiding from the Nazis more extreme area.
The first area I guess concerns broadly what we normally call white lies. Tiny dishonesties to ease social interactions and make sure nobody gets hurt.
Well, simply put, I don’t believe in them. Again, I think there’s a lot to be said for tact, and knowing what to say and what not to say, and how to focus on the positive. However, I honestly feel if you lie to me you demean me as a person, and ultimately I would rather know something if I ask for it that is the truth, as it’s the only way to help me improve in the long run.
(I’m not convinced that many examples in this case are real lies either, to be honest, as I think a lof of the time when an opinion is asked both people know that they’re going to flatter each other, so the intent to decieve is not really there, but that’s a side issue.)
The second issue, and the far harder one for an absolutist like me to answer, is the Nazi case. I’m very well aware that any principle I claim is good to hold would probably break down in the heat of the moment anyway and I would lie to save them, but that is neither here nor there - the question is, should I act that way?
Let me build up to this point by means of two routes, first by looking at why truth is valuable, and second by looking at a case in between the white lies and Nazi cases.
I believe truth is a noble thing. I believe there are things greater than life and death. I believe truth is one quality that separates us from the randomness and chaos of nature and makes us something better, gives us dignity. I believe that without there is a danger of the world becoming random, of becoming meaningless. I believe that truth leads to knowledge leads to greater understanding of the world. I believe knowing that someone will always tell me the truth is a core part of me trusting them, and I need trust and a core sense of knowledge to make sense of the world. I believe the world would be a far greater place if nobody lied in any situation, and although that’ll never be the case, it doesn’t hurt to at least set an example.
To try and understand why I would reccomend truth even if would seem to bad consequences, I’d draw an example of asking someone to deny their religious faith. Should a Christian deny their faith to save someone else’s life? To save them some pain? To save them a mild inconvenience? I think it is arguable, at the very least, and that’s a key sense of what I’m getting at here. Of course you could argue that trying to equate faith in God with faith in the truth is ridiculous, and perhaps you’re right - but people find different ways to define meaning in their lives.
So, to summarise up to here, I think there would have to be extreme, extreme reasons to even begin to think about falsehood, and that the usual best stance to take is to tell the truth. How am I judge when it is appropriate to lie? My best answer is that I feel I am likely to be a terrible (and most likely extremely biased, trying to think of the easy way out) judge in matters like this, and am the last person who should be judging. As in 99.9% of cases I feel I am inadequate to judge, not being omniscient and not having full knowledge of the details and possibilities of any situation, I default to trying to attempt to always tell the truth.
But, the sceptics would very reasonably reply, what if one of those 0.1% of cases occurs? What if the Nazi situation comes up? My answer to this is that it is almost certain that you can mislead the Nazi in other ways, by selective reples and leaving out information. Telling lies is unlikely to be neccesary.
Okay, but what if the absolute worst does happen, let’s simplify it to a man holding a gun to someone’s head, and if you don’t lie that person will definitely lie. Well, again, I repeat that I would probably break and lie at this point. However, there are a few reasons why I think you might not: firstly, you still hold yourself as an uncompetent judge and fear that if you let any exceptions into your absolute view it will become a slippery slope, and so will not even allow this; you are prepared to sacrifice the life for some higher purpose, a belief that humanity is better than this, more noble than falsehood, and that again, life/death aren’t everything.
To which the reply comes, stop being a pretentious fool, your principles aren’t worth someone’s life. Which is possibly true, but I think there’s at least an argument to be made against it, because without principles, we’re pretty much nothing, really.