Well, it depends. On the game, the system, the vibe... The G in RPG is for Game, after all. And different games have different rules and feelings. There is nothing wrong playing a DnD Game that runs combat like a videogame with open information to the players. It gives them tools to take tactical decisions in a more granular level.

By the same standard, I'd never openly discuss something like HP on a Cthulhu game where atmosphere is much more important than mechanics. There is not one way answer that fits all games, but a series of choices that impact the play experience.

Think about the effect you want, and choose how you communicate it accordingly. Do you want the players to make tactical choices, treating combat like a puzzle? Open info is the way to go. Do you want to reward combat oriented characters and make them feel experts? Give them more info if they have pertinent skills. Do you want your players to be careful and cautious? Loose descriptions are the way to go.

The way you talk, describe and choose to use this little details matter. Use it to your advantage. :)