The breakup was a good thing for other reasons, but I still regret not lying to her about what I thought of the play.

Why? Best case scenario is she keeps taking you to unenjoyable plays until you find you have to end the relationship yourself anyway or finally tell her the truth. Out of all the things in a relationship whose end was "a good thing for other reasons", one argument about whether a play was any good seems like a trivial thing to regret.

I can't favour lies as such. I am however on board with people honestly communicating the connotation that they care how you feel at the expense of the denotational literal meaning of their words.

In lies, the intention is not to soften but to deceive. So I don't even like the phrase "white lie". It's like, if you're going to stab me in the back, is it better if it's with a white knife?