Yes, its greatly important to never forget that opinions aren't at all facts. They're opinions and (particularly when it comes to artistic things) they're subjective.
It's also true that just because you have an opinion doesn't automatically mean it should be expressed. That friend who is raving about some movie (or whatever) that you hate? Volunteering that you hated it is unnecessary and corrosive. Just let your friend enjoy what they enjoy without throwing cold water all over them. Your opinion of it is no more valid than theirs. If they want your opinion, they'll ask.
It's also true that just because you have an opinion doesn't automatically mean it should be expressed. That friend who is raving about some movie (or whatever) that you hate? Volunteering that you hated it is unnecessary and corrosive. Just let your friend enjoy what they enjoy without throwing cold water all over them. Your opinion of it is no more valid than theirs. If they want your opinion, they'll ask.