Gawker gives you the rules on using sick days (hint: don't use them when you are sick because that is no fun at all).
The hangover day has been a topic of conversation. Some people feel it's OK as long as you don't do it a lot. Me? I like to punish myself for drinking too much (why is that a lesson we have to learn over and over in life?). So I work through it...sometimes I even run it off.
When I have managed people, I have always been cool with the impromptu day off. If you don't have anything on your calendar, skedaddle. No need to act sick.