I've gotten a lot of grief for telling non-Catholics the Catholic truth. Funnily enough, the grief wasn't even from the non-Catholics, but from Catholics!
All I can say is, either something is reality, or it isn't. If it is, then it is important for EVERYONE to know. If not, then nobody should believe in it or spread it.
The Catholic faith is real. It's teachings are the truth about eternal realities. It's laws are God's real laws. Even without knowing an "official Church position" my common sense tells me that if something is real, it must be important to everyone. And if it's a reality that a person's actions can land the person in hell, then all the more so!
All of my non-Catholic friends I've had in the last many years, knew I was Catholic, knew I had morals I believed were a matter of fact rather than opinion, and knew I would tell them what I thought if they were doing something messed up. As far as I know, all of them respected me for it, even if sometimes it drove them crazy, and even if they thought I was "a little overboard" (as true morality seems to most people).
If I have lost any friendship over professing the truth, that is the problem of that person. If I ever cared about them, I did well to tell them the truth, even if they hated me for it. But those were all cases wherein all I stood to loose was their friendship, while they stood to loose their soul, their health, etc.. Those are important reasons to speak up, especially when the worst that could happen is someone gets mad at you or doesn't like you anymore.
That being said, there is room for prudence in every decision. If preaching the faith gets your head blown off, in a situation where you could have lived without denying the faith by keeping silent, you probably should have kept your mouth shut. If talking about it only makes the person deeper in their hatred of God or religion, you probably should keep your mouth shut.
Truth is great, but then there's just doing ALL damage and NO good. Sometimes there ARE reasons that would make not saying anything the right thing to do in a given situation. Like with the rest of life and morality, you DO have to use your common sense, and exercise some good judgment.
Very little in life will be "always yes" or "always no" propositions. Even killing is justified in self defense. Likewise, there are times when telling the truth is prudent or even required, regardless of the cost, and times when we stand to actually hurt someone's chances of salvation more by saying something, than we would by NOT saying it.
So really, I think telling the Catholic truth about things is good in most situations, even if the cost is high to us personally (like loosing a friend or a family member not liking us anymore)... but there are situations that are exceptions to that rule, wherein you would do more damage than good. It's something you have to judge carefully from situation to situation. Also there is a question of whether you are in a position where you obliged to say something or not (like a parent to a child), or of whether you're even in a position to say something (such as the difference between walking into an atheist meeting and preaching the Faith, versus giving your honest knowledge to a good friend that respects you, whom you might actually influence.)
Important afterthought: We also must remember that ultimately each and every soul on earth is God's and in His hands. Often we THINK we can change everything, or that we have the power to influence where we really can't and don't. For the most part, the matter of salvation rests between God and the individual. We might be able to help by saying the truth, but the grace has to come from God, and the CHOICE can only be made by that person. Eg, verbally bashing them over the head with the truth repeatedly, for instance, usually doesn't help, because the mind of the individual is already made up solidly to do what they want to do and live how they want to live.