Personally, I think credit ratings scores are a scam. Your reward isn't usually more then 2-3%. Hardly enough to justify all the money wasted on interest you paid to earn it. Most banks and lenders will negotiate rates if you ask or shop around. As long as you don't have a lot of negative issues, your score doesn't really amount to much. Just pay your bills responsibly, and you can get decent rates. Mostly credit ratings are used to justify higher rates (greater profits), but most are willing to do anything to get your business. They can always slip something into the agreement, which allows them to raise their rates, or high penalties for late payments later (sneaky bastards).
Use credit only when you need it, not just to earn a basically useless and arbitrary number on paper and you'll be much better off in the long run.