Here's an axiom familiar to borrowers: Using too much of your available credit hurts your credit score. A personal finance rule of thumb that goes with it says that for a good credit score, keep your "credit utilization ratio" -- what you use versus how much you have to use -- below 30%. The rule applies to each card individually, and to the cumulative limits of all your cards.
So if you have a card line with a $10,000 limit, for the best credit score, don't carry a balance higher than $3,000. Simple, right?