I'm a 20 year player (casino-style) and a former dealer (also casino), and I can tell you that in virtually all professional cardroom situations the "show one, show all" rule exists...and yes, it is a rule, not a "courtesy". Moreover (and this seems to be a little-known rule among players AND dealers, but it is a rule nevertheless), in Texas Hold'em, if, at the river, ANY player calls the final bet, then ANY PLAYER AT THE TABLE (regardless whether he's still in the hand or not) can ask to see (and by rule will be shown) the cards of ANY PLAYER who has met that final bet. In other words, if one person bets, two people call, and the original bettor shows "the nuts", the other two players may attempt to muck their hands, but anyone at the table can request to see either or both of the two remaining hands (and will be shown them, unless the dealer/floorperson is clueless). Of course on the internet, this doesn't hold true...though it should. And as for the "strategy" of showing cards, there are two distinct schools of thought, both with merit. The first (and this is the one I agree with) is that you never EVER give more information to your opponent than you absolutely must; therefore, if you don't have to show your cards, you don't. Personally, I don't want anyone knowing what I stayed with, what I played with, what I bet with, what I raised with, what cards I'll play in a certain position, etc. A good player will most definitely use that information against you later...and cost you far more than any perceived "gain" you might make by showing. The second is that there IS something to be gained by showing on occasion...you can put a player on tilt, set a player up for later by showing the opposite of what you'll do next time, etc. Again, while this has legitimate application, I look at this as being a bad idea, mathematically. The percentage "gain" is minimal compared to the potential percentage loss. But then I'm old school...and a lot of players a lot better than me show their cards routinely.

But even good players are just stupid sometimes.