You do if you know its the kind of thing that could potentially put someone off a relationship and they might get upset that it was a lie by omission kind of thing, that you did not tell them earlier.
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“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass