The rule and ettiquitte on asking to see mucked cards at showdown?

[SammyG-SD]

I will ask sometimes, but it has to meet the situation of a maniac/shady guy who has been angle shooting; and I ask when its allin before the flop/turn is dealt. Usually the dealer just ask to both set of cards, but if the person really wants to muck, then let him. I don't think you should ever ask after the person has mucked, unless you really expect cheating.