What, in your view, and in your experience, is the nature of the interaction between honesty and rationality? Between external truthtelling and internal truthseeking?

Could it be as simple as forming a habit?

Being honest as much as possible 'by default' turns into a habit that can be very helpful when it comes to being rational. I have the feeling that external truthtelling helps form that habit, which leads to fewer internal lies. But I can't prove it (maybe it's just me).

The chain goes something like: habit of external truth -> helps internal truth, even when it's inconvenient -> helps to practice the art of rationality, because it's harder to avoid tough questions and problems by deceiving yourself.