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

[Lagtastic]

if i show the winning hand opponents can much-----fine. but what gets me is this:

the river gets checked down when its HU and i have position. the guy who is first to act must have missed a draw and just says "i muck" or "i fold" or whatever. Im gonna ask to see his cards everytime before i even show my hand. I dont think that is out of line, but i feel like alot of people do. the way i see it is, i show my cards in order when its time for a showdown. i dunno, a lil off topic but it bothers me