Your goal is to build a history of responsible use of and repayment of your credit cards.
In my analysis, the sweet spot for the FICO score machine is 20% of the available credit line.
Obviously, the algorithm places high value on prompt consistent payments.
Other key factors are the age of the credit line and the type of credit used.
In my opinion, it would be unwise to run the balances up to 50% if you are tying to build your score.
To learn more about scores and the components, go to
www.fico.com. As I recall, there is a score simulator at
www.bankrate.com where you can play with components and see what effect they have.