Rationalist position towards lying?

Rationality isn't the sort of thing that can take positions on things. But many prominent rationalist writers have discussed the subject, and in general, they take a very dim view of lying, in the usual meaning of the term. The relevant aphorism, originally from Steven Kaas and quoted in the sequences here [LW · GW]:

Promoting less than maximally accurate beliefs is an act of sabotage. Don't do it to anyone unless you'd also slash their tires.

There are corner cases; the classic thought experiment in philosophy is, if you were hiding Jews in your attic during WW2, and the Nazis came and asked if you were, you wouldn't tell them about it (and wouldn't glomarize). These are discussed here [LW · GW].

Rationalists also tend to generalize that to many cases which ordinary people wouldn't consider to be lying, where self-awareness strips away excuses and offers a higher standard to aspire to. In particular, most people by default people follow a strategy where they deceive themselves, without knowing it, so they can pass those deceptive beliefs on to others (see: The Elephant in the Brain). We also hold a dimmer-than-usual view of statements that don't try to refer to concrete facts at all, but serve other purposes; see posts on Simulacrum Levels [? · GW] for discussion of how that works and why it's bad.

Since there is no consensus definition of rationality, there cannot be definitive answers to this type of question.

If you take rationality to be maximising utility, you're still left with questions. Is it individual utility or collective utility? If it's collective utility, what is the scope of the collective: the nation or the globe?

In my opinion, for each of these types of rationality, there are marginal circumstances where lying would be the most rational course of action. It's hard to avoid this fact if you're a consequentialist, and most modern rationalists are.

To take a trivial example, lying could prevent a global catastrophe that would wipe out the human race. Of course, if you take a broader scoped morality, there are likely to be less circumstances in which lying is the rational approach than if you are maximising individual utility, but such circumstances do exist.

If you wanted to go more deeply into it, you would have to consider the fact that individual utility and collective utility are not entirely separate.