I have co-workers that ask me to come in earlier than scheduled because they're "sick" (for some reason they get sick a lot on Friday nights), I always make it a point to come in at least 30 minutes later than scheduled whenever they call me.
It's kinda of a good thing/bad thing for you; yes you have to come in earlier or cancel plans if you accept, but it guarantees extra hours for you (possibly overtime)
I used to call off work back then because I money wasn't a big concern for me; I was living with my folks, wasn't paying rent, and they still gave me an allowance for doing chores. Still irresponsible from my part, but I don't really mind. Least I didn't put work over school.
But with this very tough economy I wouldn't dream of calling in sick; crazy as it sounds I dread even losing 2 hours out of my paycheck.