the optimal mix of credit is about 2-3 visa/mastercard/charge cards AND 1-2 store cards AND then (a mortgage or car installment loan). Anything less is less than optimal. You can read about scoring procedures from the folks who make fico. 
Fairisaac is the folks that make it. They document the factors, which many of you misconceive that affect your score. You can have $100,000 in visa credit line, and have a VERY high score. 
Even if you pay your balance in full , your credit card balance reported to the bureaus is that when the statement closes. So if you pay it off the next day, it will still look as if you carry a balance.
It's wise to never "appear" to have more than 30% of your available credit line in use.
So if you charge $7500 out of $10000 limit on your visa in a month, roll a statement, and then pay it off to avoid finance charges. Your credit report will show 75% utilization. This is bad. It's important to have plenty of excess credit, to lower your utilization ratio.