Yes, you can reveal cards from your hand whenever you want.
The regular comprehensive rules don't address this one way or the other, but in the tournament rules, section 3.12 entitled "Hidden Information" says
Hidden information refers to the faces of cards and other objects at which the rules of the game and format do not allow you to look.
Throughout the match, a draft, and pregame procedures, players are responsible for keeping their cards above the level of the playing surface and for making reasonable efforts to prevent hidden information from being revealed. However, players may choose to reveal their hands or any other hidden information available to them, unless specifically prohibited by the rules. Players must not actively attempt to gain information hidden from them, but are not required to inform opponents who are accidentally revealing hidden information.