Unfortunately, exact Halachos about emes v’sheker are not in the Shulchan Aruch, hence the obvious confusion about lying.
There certainly is no Klal that one may lie for Shalom Bayis. Nor that one may not for shidduchim. One would be hard pressed to find a source in Rishonim that applies Midbar Sheker Tirchak to something which does not cause damage to another.
Beyond that, there is a concept of Hin Tzedek, of one getting used to being a truthly person. But that is not midbar sheker tirchak.
To lie in such which a way which causes damage – that is, to lie when the other person has a right to know – is forbidden. But the underlying concept here is right to know. A Talmid Chacham may lie about those things because the asker has no right to know them.
However, this gets extremely murky. What’s called a “right to know”? What’s called “damage”. Many Poskim have applied this to lie within reason about a girl’s age in shidduchim, because it is irrelevant (not 20 to 30, but what about 24 to 23?). Hence, the right to know evaporates. The waters are murky. Mishaneh Mifney Darkey Shalom – which most poskim apply to even outwardly lying if it so necessary – is a similar concept. Rabbenu Yonah states that one may not lie in such a way to cause damage to another person. Only, when the situation is truly a peaceful one, merely something irrelevant was said or done, and will destroy the legitimate and real peace (and hence there is no real knowledge to be gained) may lying be employed.
It really is case by case to decide what’s called irrelevant and no right to know. If a boy smoked in high school, is that relevant, and has now quit. Or if his family uses plastic? May one lie? The waters are murky, indeed. Hence the happy usage of a local orthodox Rabbi, or at the very least, Husband.