As stated correctly by the other answers, it's perfectly legal to reveal cards from your hand to your opponents at any time. As the question seems to be more about possible situations where such action may be misinterpreted, let's delve deeper into that question.
Basically, whenever you reveal cards from your hand for a clearly social reason (such as suggesting you have a sure way to win, or bragging about a top-deck draw), you should be fine, as you're likely to accompany this by some kind of commentary or body language. It gets more complicated when you're expected to do something, such as cast a spell or reveal a card, and you choose to reveal any unrelated card instead of doing that.
As an extreme example to illustrate the concept, let's say you just entered your main phase, tapped an exhaustive amount of mana, then reveal All Is Dust (or another card that requires no further interaction, such as choosing targets) without commenting it. An opponent then responds with tapping two Islands and casts Counterspell. Surely, you will have a hard time explaining to that player that you really just meant to show them the card and weren't actually casting it, and I'm sure in a tournament scenario, you wouldn't get away with it, no matter how much the rules cover your action.
Something more encounterable would be Keranos, God of Storms and players misinterpret revealing the card because of Keranos's ability is actually casting that card (which makes sense, considering players rarely do something in their draw step after drawing a card for the turn), causing them to act as if the spell would've been cast and just resolved. This might happen because they're distracted, Keranos's controller not being clear enough, or a combination of the two. It mostly won't be a problem, but might sometimes result in them revealing choices they'd make, which obviously gives the Keranos player a great advantage not only because they now have better information regarding that spell specifically, but also where to direct the 3 damage Keranos deals.
As with many things in MTG, rules alone only define what's allowed, but you will have to make an effort to communicate your actions in an
ambiguous or interpretable situation. These examples cover extreme cases that probably won't appear in any tournament scenario - which isn't by chance, but because players usually have a good sense of when their actions might be confusing others.