I don't understand why everyone gets in a huff about asking to see mucked hands. If you have paid to be in the showdown, then you technically must show. Mucking a showdown hand is affected behavior.
Seeing hole cards allows you to see what kind of range someone is playing, and that's an advantage that I can legally have. I don't see why people get so pissy about this. They have no real basis grounded in any rules for being mad.
Take it for what it is worth.....It is generally considered one of the lowest things you can do in live poker (thought of like this, you won the money so take the pot like a gentleman and do not embarrass or belittle the other player by asking to see his cards). You can generally get away with it sparingly if there is a good reason (like the other guy is an a*hole to the table).
The Gods of Poker just do not want you doing it. They even have a rule enforced by almost all
casinos, in that if you ask the players hand to be shown, and it is a winner (other player overlooked his hand), the other player is awarded the pot even if it is mucked, dead (I have to admit I have never ever seen a player win that way, but I have heard stories of it).
If you do it, be prepared to be the object of scorn and dislike by not only the player whose cards you ask to see, but usually the entire table. It is truly akin to fighting words (show me his cards please) and I have seen players come close to blows on more than one occasion in the situation. In general, the player you do this to will be incensed (especially an experienced live player) and will be looking to stick it to you in a more focused way.
If you know what you are doing, and you want to put someone on tilt, it is the perfect weapon, if you can stand the increased heat from the person and the entire table who will also be watching you more carefully, and may try to take a shot at you as well.
It is the object of many poker threads, and high emotion usually inhabits these threads.
In summary, it is thought of as extremely poor poker etiquette and it should really just never be done, except in extra rare circumstances.
P.S. Not that it matters, but I am among the long list of players that will be severely pissed if someone asks to see the hand that I mucked.