Do we mean "truth" as in "true facts?" So the question is really, "is it okay to lie about certain facts if you think it will preserve hope?"
Definitely not, for two reasons: (1) it may not have the intended effect on hope levels, and (2) that kind of manipulation is never helpful in the long run (WTF happens when they find out that you lied and that they're basing their hope on shit that's just not true? Dummy.)
I don't even care if hope is possibly true. It's just not in me to point somewhere in the horizon. I'm dealing with this in a family member who's been depressed.. they're very focused and aggravated on things not changing externally or being in order or meeting some expectation. Then they drink and dwell some more. They need changes, but I think they could simply do with better coping methods. I've worked on that myself. Pay attention to what is here, not to what isn't. Hope just keeps pointing to what isn't. It's just acknowledging they need more...and more and more. Usually the truth is we have it pretty good. And if I know that for sure with someone, I'm gonna point it out.