I guess you have to know the person well in the relationship. Since one has to guess how another will respond in any situation, one can only judge oneself and if the intent is pure, better to keep the secret or truth, and if eventually found out the deceiver must explain the good intent.
I'm wary of couples who insist on total honesty in all things. Especially if one of the partners isn't self reflective at all.
RE: The Webs We Weave: Freedom and the Common Good