Recently in CA, an office manager from a lending group we work with, advised a client looking to get refinanced, to 100% pay off an $8,000 line that was maxed out. They paid the $8,000 but did not close the account, and when the scoring system then addressed the $8,000 of available new credit, their score went down almost 20 points.
Using the FICO software we use to work with our clients (when we later put this client's credit history into the system), it told us that they would have increased their score the 9 points they needed by paying down the $8,000 line to 78% of available balance (from the high balance of $7,974 that it was) rather than by paying it off completely. It's sometimes better to carry a balance and keep current on payments than to have a whole lot of new credit available that might then compete with a new loan obligation.
We told them to use the card and bring the balance back to the recommended $6,140.00, which they did. They made their next minimum payment and the score adjusted 12 points above where it was before, and 3 points more than what they needed for their refinanced loan.