The trouble with telling the truth all of the time is that you can't tell the truth all of the time. One of my all-time favorite steam room stories illustrates this.
Walking into the steam room, all I can smell is vinegar with a strong hint of something else that I can't pin my nose on. Without my glasses and with a lot of steam, everything looks like an impressionistic painting, but I can see enough to see a woman with a container next to her who is rubbing her legs with a homemade mixture of something. The smell of the steam room indicated the homemade part as this smell wasn't something anyone would buy. For perspective, running into women with homemade exfoliation recipes isn't uncommon. Its why you really need to check out my gym.
Me: Can I ask, is it vinegar that I am smelling?
Her: Yes, I can't believe you noticed. I hope it isn't bothersome.
Me: No, not at all. I really love the smell if vinegar. I smell something else. What did you mix with the vinegar?
Her: It's going to sound kind of odd, but I found some expired Dijon mustard, so I used Dijon. I hope it isn't bothersome to you.
Me: Oh, no... not at all. I really love the smell of Dijon too.
I didn't respond truthfully because the truth wasn't going to make anything better. It made me think of all of the times I ask Chris, "Do these pants look good?" or "Do these pants make my butt look big?". When I ask these questions, I am not looking for the truth. I really am not. Sometimes the bold truth isn't what someone needs or deserves. Here are some good examples of when you can tell someone the "Expired Mustard & Vinegar" (un)truth...
When you need to protect someone's feelings
To maintain privacy
To avoid conflict that isn't necessary or that you aren't ready for
To protect yourself or others
To maintain professional relationships