Here's my understanding...
You won't HURT your credit score unless you get to past due status. Credit card companies call people who pay off their cards "dead beats" because they aren't making any money off of them:D :D
Having more charged on your cards will hurt your "available credit" when a company looks to see how much you owe and what your monthly commitments are. The less you owe the better.
However, I have heard that it's better for your credit (we're improving either way) to have a history of slowly paying off a card over time than to pay it all at once. Don't know whether that's true or not, and it's certainly never stopped me from paying the card when I could - I don't want the possible small benefit in credit score enough to pay the finance charges....
Now my question: Will it hurt my credit score to cancel my "oldest" credit card? I don't carry a balance on anything now, so I'm not worried about harming my available credit, but I am concerned about shortening my "credit history" and losing ground that way...